Understanding IP Addressing:
Everything You Ever Wanted To Know
The Internet continues to grow at a phenomenal rate. This is reflected in
the tremendous popularity of the World Wide Web (WWW), the opportunities
that businesses see in reaching customers from virtual storefronts,
and the emergence of new ways of doing business. It is clear that expanding
business and public awareness will continue to increase demand for access
to resources on the Internet.
Internet Scaling Problems
Over the past few years, the Internet has experienced two major scaling
issues as it has struggled to provide continuous and uninterrupted
growth:
• The eventual exhaustion of IP version 4 (IPv4) address space
• The need to route traffic between the ever increasing number of networks
that comprise the Internet
The first problem is concerned with the eventual depletion of the IP
address space. IPv4 defines a 32-bit address which means that there are
only 232 (4,294,967,296) IPv4 addresses available. As the Internet continues
to grow, this finite number of IP addresses will eventually be
exhausted.
The address shortage problem is aggravated by the fact that portions of
the IP address space have not been efficiently allocated. Also, the traditional
model of classful addressing does not allow the address space to
be used to its maximum potential. The Address Lifetime Expectancy
(ALE) Working Group of the Internet Engineering Task Force (IETF) has
expressed concerns that if the current address allocation policies are not
modified, the Internet will experience a near to medium term exhaustion
of its unallocated address pool. If the Internet’s address supply
problem is not solved, new users may be unable to connect to the global
Internet. More than half of all possible IPv4 addresses have been
assigned to ISPs, corporations, and government agencies, but only an
estimated 69 million addresses are actually in use.
1
FIGURE 1. Network Number Growth
The second problem is caused by the rapid growth in the size of the
Internet routing tables. Internet backbone routers are required to maintain
complete routing information for the Internet. Over recent years,
routing tables have experienced exponential growth as increasing numbers
of organizations connect to the Internet. In December 1990 there
were 2,190 routes, in December 1995 there were more than 30,000
routes, and in December 2000 more than 100,000 routes.
Unfortunately, the routing problem cannot be solved by simply
installing more router memory and increasing the size of the routing
tables. Other factors related to the capacity problem include the growing
demand for CPU horsepower to compute routing table/topology
changes, the increasingly dynamic nature of WWW connections and
their effect on router forwarding caches, and the sheer volume of information
that needs to be managed by people and machines. If the number
of entries in the global routing table is allowed to increase without
bounds, core routers will be forced to drop routes and portions of the
Internet will become unreachable.
The long-term solution to these problems can be found in the widespread
deployment of IP Next Generation (IPng or IPv6). Currently,
IPv6 is being tested and implemented on the 6Bone network, which is
an informal collaborative project covering North America, Europe, and
Japan. 6Bone supports the routing of IPv6 packets, since that function
has not yet been integrated into many production routers. Until IPv6
can be deployed worldwide, IPv4 patches will need to be used and
modified to continue to provide the universal connectivity users have
come to expect.
UNDERSTANDING IP ADDRESSING 2
FIGURE 2. Growth of Internet Routing Tables
Classful IP Addressing
When IP was first standardized in September 1981, the specification
required that each system attached to an IP-based Internet be assigned
a unique, 32-bit Internet address value. Systems that have interfaces to
more than one network require a unique IP address for each network
interface. The first part of an Internet address identifies the network on
which the host resides, while the second part identifies the particular
host on the given network. This creates the two-level addressing hierarchy
that is illustrated in Figure 3.
In recent years, the network number field has been referred to as the
network prefix because the leading portion of each IP address identifies
the network number. All hosts on a given network share the same network
prefix but must have a unique host number. Similarly, any two
hosts on different networks must have different network prefixes but
may have the same host number.
Primary Address Classes
To provide the flexibility required to support networks of varying sizes,
the Internet designers decided that the IP address space should be
divided into three address classes-Class A, Class B, and Class C. This is
often referred to as classful addressing. Each class fixes the boundary
between the network prefix and the host number at a different point
within the 32-bit address. The formats of the fundamental address
classes are illustrated in Figure 4.
3
FIGURE 3. Two-Level Internet Address Structure
FIGURE 4. Principle Classful IP Address Formats
One of the fundamental features of classful IP addressing is that each
address contains a self-encoding key that identifies the dividing point
between the network prefix and the host number. For example, if the
first two bits of an IP address are 1-0, the dividing point falls between
the 15th and 16th bits. This simplified the routing system during the
early years of the Internet because the original routing protocols did not
supply a deciphering key or mask with each route to identify the length
of the network prefix.
Class A Networks (/8 Prefixes)
Each Class A network address has an 8-bit network prefix, with the
highest order bit set to 0 (zero) and a 7-bit network number, followed
by a 24-bit host number. Today, Class A networks are referred to as
“/8s” (pronounced “slash eight” or just “eights”) since they have an 8-
bit network prefix.
A maximum of 126 (27 -2) /8 networks can be defined. The calculation
subtracts two because the /8 network 0.0.0.0 is reserved for use as the
default route and the /8 network 127.0.0.0 (also written 127/8 or
127.0.0.0/8) is reserved for the “loopback” function. Each /8 supports a
maximum of 224 -2
(16,777,214) hosts per network. The host calculation subtracts two
because the all-0s (all zeros or “this network”) and all-1s (all ones or
“broadcast”) host numbers may not be assigned to individual hosts.
Since the /8 address block contains 231 (2,147,483,648 ) individual
addresses and the IPv4 address space contains a maximum of 232
(4,294,967,296) addresses, the /8 address space is 50 percent of the total
IPv4 unicast address space.
Class B Networks (/16 Prefixes)
Each Class B network address has a 16-bit network prefix, with the two
highest order bits set to 1-0 and a 14-bit network number, followed by a
16-bit host number. Class B networks are now referred to as “/16s” since
they have a 16-bit network prefix.
A maximum of 16,384 (214 ) /16 networks can be defined with up to
65,534 (216-2) hosts per network. Since the entire /16 address block
contains 230 (1,073,741,824) addresses, it represents 25 percent of the
total IPv4 unicast address space.
Class C Networks (/24 Prefixes)
Each Class C network address has a 24-bit network prefix, with the
three highest order bits set to 1-1-0 and a 21-bit network number, followed
by an 8-bit host number. Class C networks are now referred to as
“/24s” since they have a 24-bit network prefix.
A maximum of 2,097,152 (221 ) /24 networks can be defined with up to
254 (28-2) hosts per network. Since the entire /24 address block contains
229 (536,870,912) addresses, it represents 12.5 percent (or oneeighth)
of the total IPv4 unicast address space.
UNDERSTANDING IP ADDRESSING 4
Other Classes
In addition to the three most popular classes, there are two additional
classes. Class D addresses have their leading four bits set to 1-1-1-0 and
are used to support IP Multicasting. Class E addresses have their leading
four bits set to 1-1-1-1 and are reserved for experimental use.
Dotted-Decimal Notation
To make Internet addresses easier for people to read and write, IP
addresses are often expressed as four decimal numbers, each separated
by a dot. This format is called “dotted-decimal notation.”
Dotted-decimal notation divides the 32-bit Internet address into four 8-
bit fields and specifies the value of each field independently as a decimal
number with the fields separated by dots. Figure 5 shows how a
typical /16 (Class B) Internet address can be expressed in dotted-decimal
notation.
Table 1 displays the range of dotted-decimal values that can be assigned
to each of the three principle address classes. The “xxx” represents the
host number field of the address that is assigned by the local network
administrator.
5
FIGURE 5. Dotted Decimal Notation
TABLE 1. Dotted Decimal Ranges for Each Address Class
Unforeseen Limitations to Classful Addressing
The original Internet designers never envisioned that the Internet
would grow into what it has become today. Many of the problems that
the Internet is facing today can be traced back to the early decisions
that were made during its formative years.
• During the early days of the Internet, the seemingly unlimited
address space allowed IP addresses to be allocated to an organization
based on its request rather than its actual need. As a result, addresses
were freely assigned to those who asked for them without concerns
about the eventual depletion of the IP address space.
• The decision to standardize on a 32-bit address space meant that there
were only 232 (4,294,967,296) IPv4 addresses available. A decision to
support a slightly larger address space would have exponentially
increased the number of addresses thus eliminating the current
address shortage problem.
• The classful A, B, and C octet boundaries were easy to understand
and implement, but they did not foster the efficient allocation of a
finite address space. Problems resulted from the lack of a network
class that was designed to support medium-sized organizations. For
example, a /24, which supports 254 hosts, is too small while a /16,
which supports 65,534 hosts, is too large. In the past, sites with several
hundred hosts were assigned a single /16 address instead of two
/24 addresses. This resulted in a premature depletion of the /16 network
address space. Now the only readily available addresses for
medium-sized organizations are /24s, which have the potentially negative
impact of increasing the size of the global Internet’s routing table.
Figure 6 shows basic class A, B, and C networks.
UNDERSTANDING IP ADDRESSING 6
The subsequent history of Internet addressing involved a series of steps
that overcame these addressing issues and supported the growth of the
global Internet.
Additional Practice with Classful Addressing
Appendix B provides exercises using Classful IP Addressing.
7
FIGURE 6. Basic Class A, B, and C Networks
UNDERSTANDING IP ADDRESSING 8
Subnetting
In 1985, RFC 950 defined a standard procedure to support the subnetting,
or division, of a single Class A, B, or C network number into
smaller pieces. Subnetting was introduced to overcome some of the
problems that parts of the Internet were beginning to experience with
the classful two-level addressing hierarchy, such as:
• Internet routing tables were beginning to grow.
• Local administrators had to request another network number from the
Internet before a new network could be installed at their site.
Both of these problems were attacked by adding another level of hierarchy
to the IP addressing structure. Instead of the classful two-level hierarchy,
subnetting supports a three-level hierarchy. Figure 7 illustrates
the basic idea of subnetting, which is to divide the standard classful
host number field into two parts-the subnet number and the host number
on that subnet.
Subnetting attacked the expanding routing table problem by ensuring
that the subnet structure of a network is never visible outside of the
organization’s private network. The route from the Internet to any subnet
of a given IP address is the same, no matter which subnet the destination
host is on. This is because all subnets of a given network number
use the same network prefix but different subnet numbers. The routers
within the private organization need to differentiate between the individual
subnets, but as far as the Internet routers are concerned, all of
the subnets in the organization are collected into a single routing table
entry. This allows the local administrator to introduce arbitrary complexity
into the private network without affecting the size of the Internet’s
routing tables.
Subnetting overcame the registered number issue by assigning each
organization one (or at most a few) network numbers from the IPv4
address space. The organization was then free to assign a distinct subnetwork
number for each of its internal networks. This allowed the
organization to deploy additional subnets without obtaining a new network
number from the Internet.
FIGURE 7. Subnet Address Hierarchy
In Figure 8, a site with several logical networks uses subnet addressing
with a single /16 (Class B) network address. The router accepts all traffic
from the Internet addressed to network 130.5.0.0, and forwards traffic
to the interior subnetworks based on the third octet of the classful
address. The deployment of subnetting within the private network provides
several benefits:
• The size of the global Internet routing table does not grow because
the site administrator does not need to obtain additional address space
and the routing advertisements for all of the subnets are combined
into a single routing table entry.
• The local administrator has the flexibility to deploy additional subnets
without obtaining a new network number from the Internet.
• Route flapping (that is, the rapid changing of routes) within the private
network does not affect the Internet routing table since Internet
routers do not know about the reachability of the individual subnetsthey
just know about the reachability of the parent network number.
Extended Network Prefix
Internet routers use only the network prefix of the destination address
to route traffic to a subnetted environment. Routers within the subnetted
environment use the extended network prefix to route traffic
between the individual subnets. The extended network prefix is composed
of the classful network prefix and the subnet number.
9
FIGURE 9. Extended Network Prefix
FIGURE 8. Subnetting the Routing Requirements of the
Internet
UNDERSTANDING IP ADDRESSING 10
The extended network prefix has traditionally been identified by the
subnet mask. For example, if an administrator has the /16 address of
130.5.0.0 and wants to use the entire third octet to represent the subnet
number, the administrator must specify a subnet mask of 255.255.255.0.
The bits in the subnet mask and the Internet address have a one to one
correspondence. The bits of the subnet mask are set to 1 (one) if the system
examining the address should treat the corresponding bit in the IP
address as part of the extended network prefix. The bits in the mask are
set to 0 (zero) if the system should treat the bit as part of the host number.
This numbering is illustrated in Figure 10.
The standards describing modern routing protocols often refer to the
extended network prefix length rather than the subnet mask. The prefix
length is equal to the number of contiguous one-bits in the traditional
subnet mask. This means that specifying the network address
130.5.5.25 with a subnet mask of 255.255.255.0 can also be expressed as
130.5.5.25/24. The /<prefix length> notation is more compact and easier
to understand than writing out the mask in its traditional dotteddecimal
format. This is illustrated in Figure 11.
Note that modern routing protocols still carry the subnet mask. None of
the Internet standard routing protocols have a 1-byte field in the header
that contains the number of bits in the extended network prefix. Each
routing protocol is still required to carry the complete four-octet subnet
mask.
FIGURE 10. Subnet Mask
FIGURE 11. Extended Network Prefix Length
11
Subnet Design Considerations
The deployment of an addressing plan requires careful thought. Four
key questions that must be answered before any design should be
undertaken are:
1 How many total subnets does the organization need today?
2 How many total subnets will the organization need in the future?
3 How many hosts are on the organization’s largest subnet today?
4 How many hosts will there be on the organization’s largest subnet in
the future?
The first step in the planning process is to take the maximum number of
subnets required and round up to the nearest power of two. For example,
if an organization needs nine subnets, 23 (or 8) will not provide
enough subnet addressing space, so the network administrator will
need to round up to 24 (or 16).
The network administrator must always allow adequate room for
growth. For example, although 14 subnets are required today, 16 subnets
might not be enough in two years when the 17th subnet needs to
be deployed. In this case, it would be wise to select 25 (or 32) as the
maximum number of subnets.
The second step is to ensure that there are enough host addresses for
the organization’s largest subnet. If the largest subnet needs to support
50 host addresses today, 25 (or 32) will not provide enough host address
space so the network administrator will need to round up to 26 (or 64).
The final step is to make sure that the organization’s address allocation
provides enough bits to deploy the required subnet addressing plan.
For example, if the organization has a single /16, it could easily deploy 4
bits for the subnet number and 6 bits for the host number. However, if
the organization has several /24s and it needs to deploy nine subnets, it
may have to subnet each of its /24s into four subnets (using 2 bits) and
then build the network by combining the subnets of three /24 network
numbers.
An alternative solution would be to deploy network numbers from the
private address space (RFC 1918) for internal connectivity and use a
Network Address Translator (NAT) to provide external Internet access.
Subnet Example #1
Given
An organization is assigned the network number 193.1.1.0/24 and it
needs to define six subnets. The largest subnet is required to support 25
hosts.
UNDERSTANDING IP ADDRESSING 12
Defining the Subnet Mask / Extended Prefix Length
The first step in defining the subnet mask is to determine the number of
bits required to define the six subnets. Since a network address can
only be subnetted along binary boundaries, subnets must be created in
blocks of powers of two [2 (21), 4 (22), 8 (23), 16 (24), and so on]. Thus,
it is impossible to define an IP address block such that it contains
exactly six subnets. For this example, the network administrator must
define a block of 8 (23) and have two unused subnets that can be
reserved for future growth.
Since 8 = 23, three bits are required to enumerate the eight subnets in
the block. In this example, the organization is subnetting a /24 so it will
need three more bits, or a /27, as the extended network prefix. A 27-bit
extended network prefix can be expressed in dotted-decimal notation
as 255.255.255.224. This notation is illustrated in Figure 12.
A 27-bit extended network prefix leaves 5 bits to define host addresses
on each subnet. This means that each subnetwork with a 27-bit prefix
represents a contiguous block of 25 (32) individual IP addresses. However,
since the all-0s and all-1s host addresses cannot be allocated, there
are 30 (25-2) assignable host addresses on each subnet.
Defining the Subnet Numbers
The eight subnets will be numbered 0 through 7. Throughout the
remainder of this paper, the XXX notation indicates the binary representation
of the number. The 3-bit binary representation of the decimal
values 0 through 7 are: 0 (000 ), 1 (001 ), 2 (010 ), 3 (011 ), 4 (100 ), 5
(101 ), 6 (110 ), and 7 (111 ).
In general, to define Subnet #N, the network administrator places the
binary representation of N into the bits of the subnet number field. For
example, to define Subnet #6, the network administrator simply places
the binary representation of 6 (110 ) into the 3 bits of the subnet number
field.
FIGURE 12. Example #1-Defining the Subnet
Mask/Extended Prefix Length
13
The eight subnet numbers for this example are listed in the following
code sample. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 3 bits representing
the subnet number field:
Base Net: 11000001.00000001.00000001 .00000000 = 193.1.1.0/24
Subnet #0: 11000001.00000001.00000001.000 00000 = 193.1.1.0/27
Subnet #1: 11000001.00000001.00000001.001 00000 = 193.1.1.32/27
Subnet #2: 11000001.00000001.00000001.010 00000 = 193.1.1.64/27
Subnet #3: 11000001.00000001.00000001.011 00000 = 193.1.1.96/27
Subnet #4: 11000001.00000001.00000001.100 00000 = 193.1.1.128/27
Subnet #5: 11000001.00000001.00000001.101 00000 = 193.1.1.160/27
Subnet #6: 11000001.00000001.00000001.110 00000 = 193.1.1.192/27
Subnet #7: 11000001.00000001.00000001.111 00000 = 193.1.1.224/27
An easy way to verify that the subnets are correct is to ensure that they
are all multiples of the Subnet #1 address. In this example, all subnets
are multiples of 32: 0, 32, 64, 96, and so on.
The All-0s Subnet and All-1s Subnet
When subnetting was first defined in RFC 950, it prohibited the use of
the all-0s and the all-1s subnets. The reason for this restriction was to
eliminate situations that could potentially confuse a classful router.
Today a router can be both classless and classful at the same time-it
could be running RIP-1 (classful protocol) and BGP-4 (Border Gateway
Protocol Version 4-a classless protocol) at the same time.
With respect to the all-0s subnet, a router requires that each routing
table update include the route/<prefix length> pair to differentiate
between a route to the all-0s subnet and a route to the entire network.
For example, when using RIP-1which does not supply a mask or prefix
length with each route, the routing advertisements for subnet
193.1.1.0/27 and for network 193.1.1.0/24 are identical-193.1.1.0. Without
somehow knowing the prefix length or mask, a router cannot tell
the difference between a route to the all-0s subnet and the route to the
entire network. This example is illustrated in Figure 13.
FIGURE 13. Differentiating Between a Route to the All-0s
Subnet and the Entire Network
UNDERSTANDING IP ADDRESSING 14
Regarding the all-1s subnet, a router requires that each routing table
entry include the prefix length so that it can determine whether a
broadcast (directed or all-subnets) should be sent only to the all-1s subnet
or to the entire network. For example, when the routing table does
not contain a mask or prefix length for each route, confusion can occur
because the same broadcast address (193.1.1.255) is used for both the
entire network 193.1.1.0/24 and the all-1s subnet 193.1.1.224/27. This
issue is illustrated in Figure 14.
Defining Host Addresses for Each Subnet
According to Internet practices, the host number field of an IP address
cannot contain all 0-bits or all 1-bits. The all-0s host number identifies
the base network (or subnetwork) number, while the all-1s host number
represents the broadcast address for the network (or subnetwork).
In our current example, there are 5 bits in the host number field of each
subnet address. This means that each subnet represents a block of 30
host addresses (25 -2 = 30, note that the 2 is subtracted because the
all-0s and the all-1s host addresses cannot be used). The hosts on each
subnet are numbered 1 through 30.
In general, to define the address assigned to Host #N of a particular
subnet, the network administrator places the binary representation of N
into the subnet’s host number field. For example, to define the address
assigned to Host #15 on Subnet #2, the network administrator simply
places the binary representation of 15 (011112 ) into the 5-bits of Subnet
#2’s host number field.
FIGURE 14. Identifying a Broadcast to the All 1s Subnet
and the Entire Network
The valid host addresses for Subnet #2 in this example are listed in the
following sample code. The underlined portion of each address identifies
the extended network prefix, while the bold digits identify the 5-
bit host number field:
Subnet #2: 11000001.00000001.00000001.010 00000 = 193.1.1.64/27
Host #1: 11000001.00000001.00000001.010 00001 = 193.1.1.65/27
Host #2: 11000001.00000001.00000001.010 00010 = 193.1.1.66/27
Host #3: 11000001.00000001.00000001.010 00011 = 193.1.1.67/27
Host #4: 11000001.00000001.00000001.010 00100 = 193.1.1.68/27
Host #5: 11000001.00000001.00000001.010 00101 = 193.1.1.69/27
.
.
Host #15: 11000001.00000001.00000001.010 01111 = 193.1.1.79/27
Host #16: 11000001.00000001.00000001.010 10000 = 193.1.1.80/27
.
.
Host #27: 11000001.00000001.00000001.010 11011 = 193.1.1.91/27
Host #28: 11000001.00000001.00000001.010 11100 = 193.1.1.92/27
Host #29: 11000001.00000001.00000001.010 11101 = 193.1.1.93/27
Host #30: 11000001.00000001.00000001.010 11110 = 193.1.1.94/27
The valid host addresses for Subnet #6 are listed in the following sample
code. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 5-bit host
number field:
Subnet #6: 11000001.00000001.00000001.110 00000 = 193.1.1.192/27
Host #1: 11000001.00000001.00000001.110 00001 = 193.1.1.193/27
Host #2: 11000001.00000001.00000001.110 00010 = 193.1.1.194/27
Host #3: 11000001.00000001.00000001.110 00011 = 193.1.1.195/27
Host #4: 11000001.00000001.00000001.110 00100 = 193.1.1.196/27
Host #5: 11000001.00000001.00000001.110 00101 = 193.1.1.197/27
.
.
Host #15: 11000001.00000001.00000001.110 01111 = 193.1.1.207/27
Host #16: 11000001.00000001.00000001.110 10000 = 193.1.1.208/27
.
.
Host #27: 11000001.00000001.00000001.110 11011 = 193.1.1.219/27
Host #28: 11000001.00000001.00000001.110 11100 = 193.1.1.220/27
Host #29: 11000001.00000001.00000001.110 11101 = 193.1.1.221/27
Host #30: 11000001.00000001.00000001.110 11110 = 193.1.1.222/27
Defining the Broadcast Address for Each Subnet
The broadcast address for Subnet #2 is the all-1s host address or:
11000001.00000001.00000001.010 11111 = 193.1.1.95
Note that the broadcast address for Subnet #2 is exactly one less than
the base address for Subnet #3 (193.1.1.96). This is always the case-the
broadcast address for Subnet #n is one less than the base address for
Subnet #(n+1).
15
UNDERSTANDING IP ADDRESSING 16
The broadcast address for Subnet #6 is simply the all-1s host address
or:
11000001.00000001.00000001.110 11111 = 193.1.1.223
Again, the broadcast address for Subnet #6 is exactly one less than the
base address for Subnet #7 (193.1.1.224).
Subnet Example #2
Given
An organization is assigned the network number 140.25.0.0/16 and it
must create a set of subnets that supports up to 60 hosts on each subnet.
Defining the Subnet Mask / Extended Prefix Length
The first step is to determine the number of bits required to define 60
hosts on each subnet. Since a block of host addresses can only be
assigned along binary boundaries, host address blocks can only be created
in powers of two. This means that it is impossible to create a block
that contains exactly 60 host addresses.
To support 60 hosts, the network administrator must define a minimum
address block of 62 (26-2) host addresses. However, this choice would
only provide two unused host addresses on each subnet for future
growth, which is not likely to support additional growth. The network
administrator must define a block of 126 (27-2) host addresses with 66
addresses on each subnet for future growth. A block of 126 host
addresses requires 7 bits in the host number field.
The next step is to determine the subnet mask/extended prefix length.
Since 7 bits of the 32-bit IP address are required for the host number
field, the extended prefix must be a /25 (25 = 32-7). A 25-bit extended
network prefix can be expressed in dotted-decimal notation as
255.255.255.128. This notation is illustrated in Figure 15.
FIGURE 15. Example #2-Defining the Subnet
Mask/Extended Prefix Length
17
Figure 15 shows that the 25-bit extended prefix assigns 9 bits to the
subnet number field. Since 29 = 512, nine bits allow the definition of
512 subnets. Depending on the organization’s requirements, the network
administrator could have elected to assign additional bits to the
host number field (allowing more hosts on each subnet) and reduce the
number of bits in the subnet number field (decreasing the total number
of subnets that can be defined).
Although this example creates a rather large number of subnets, it illustrates
what happens to the dotted- decimal representation of a subnet
address when the subnet number bits extend across an octet boundary.
Note that the same type of confusion can occur when the host number
bits extend across an octet boundary.
Defining Each of the Subnet Numbers
The 512 subnets will be numbered 0 through 511. The 9-bit binary representation
of the decimal values 0 through 511 are: 0 (0000000002 ), 1
(0000000012 ), 2 (0000000102 ), 3 (0000000112 ), ..., 511 (1111111112 ).
To define Subnet #3, the network administrator places the binary representation
of 3 (0000000112 ) into the 9 bits of the subnet number
field. The 512 subnet numbers for this example are listed in the following
sample code. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 9 bits representing
the subnet number field:
Base Net: 10001100.00011001 .00000000.00000000 = 140.25.0.0/16
Subnet #0: 10001100.00011001.00000000.0 0000000 = 140.25.0.0/25
Subnet #1: 10001100.00011001.00000000.1 0000000 = 140.25.0.128/25
Subnet #2: 10001100.00011001.00000001.0 0000000 = 140.25.1.0/25
Subnet #3: 10001100.00011001.00000001.1 0000000 = 140.25.1.128/25
Subnet #4: 10001100.00011001.00000010.0 0000000 = 140.25.2.0/25
Subnet #5: 10001100.00011001.00000010.1 0000000 = 140.25.2.128/25
Subnet #6: 10001100.00011001.00000011.0 0000000 = 140.25.3.0/25
Subnet #7: 10001100.00011001.00000011.1 0000000 = 140.25.3.128/25
Subnet #8: 10001100.00011001.00000100.0 0000000 = 140.25.4.0/25
Subnet #9: 10001100.00011001.00000100.1 0000000 = 140.25.4.128/25
.
.
Subnet #510: 10001100.00011001.11111111.0 0000000 = 140.25.255.0/25
Subnet #511: 10001100.00011001.11111111.1 0000000 = 140.25.255.128/25
Note that the sequential subnet numbers are not sequential when
expressed in dotted-decimal notation. This can be confusing to people
who expect dotted-decimal notation to make IP addressing easier. In
this example, the dotted-decimal notation obscures the subnet numbering
scheme.
UNDERSTANDING IP ADDRESSING 18
Defining Host Addresses for Each Subnet
In this example there are 7 bits in the host number field of each subnet
address, which means that each subnet represents a block of 126 host
addresses. The hosts on each subnet are numbered 1 through 126.
The valid host addresses for Subnet #3 are listed in the following sample
code. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 7-bit host
number field:
Subnet #3: 10001100.00011001.00000001.1 0000000 = 140.25.1.128/25
Host #1: 10001100.00011001.00000001.1 0000001 = 140.25.1.129/25
Host #2: 10001100.00011001.00000001.1 0000010 = 140.25.1.130/25
Host #3: 10001100.00011001.00000001.1 0000011 = 140.25.1.131/25
Host #4: 10001100.00011001.00000001.1 0000100 = 140.25.1.132/25
Host #5: 10001100.00011001.00000001.1 0000101 = 140.25.1.133/25
Host #6: 10001100.00011001.00000001.1 0000110 = 140.25.1.134/25
.
.
Host #62: 10001100.00011001.00000001.1 0111110 = 140.25.1.190/25
Host #63: 10001100.00011001.00000001.1 0111111 = 140.25.1.191/25
Host #64: 10001100.00011001.00000001.1 1000000 = 140.25.1.192/25
Host #65: 10001100.00011001.00000001.1 1000001 = 140.25.1.193/25
.
.
Host #123: 10001100.00011001.00000001.1 1111011 = 140.25.1.251/25
Host #124: 10001100.00011001.00000001.1 1111100 = 140.25.1.252/25
Host #125: 10001100.00011001.00000001.1 1111101 = 140.25.1.253/25
Host #126: 10001100.00011001.00000001.1 1111110 = 140.25.1.254/25
Defining the Broadcast Address for Each Subnet
The broadcast address for Subnet #3 is the all-1s host address or:
10001100.00011001.00000001.1 1111111 = 140.25.1.255
The broadcast address for Subnet #3 is exactly one less than the base
address for Subnet #4 (140.25.2.0).
Additional Practice with Subnetworks
Appendix C provides exercises using subnetting.
19
Variable Length Subnet Masks (VLSM)
In 1987, RFC 1009 specified how a subnetted network could use more
than one subnet mask. When an IP network is assigned more than one
subnet mask, it is considered a network with (VLSM) since the
extended network prefixes have different lengths.
RIP-1 Permits Only a Single Subnet Mask
When using RIP-1, subnet masks have to be uniform across the entire
network prefix. RIP-1 allows only a single subnet mask to be used
within each network number because it does not provide subnet mask
information as part of its routing table update messages. In the absence
of this information, RIP-1 is forced to make assumptions about the mask
that should be applied to any of its learned routes.
How does a RIP-1 based router know what mask to apply to a route
when it learns a new route from a neighbor? If the router has a subnet
of the same network number assigned to a local interface, it assumes
that the learned subnetwork was defined using the same mask as the
locally configured interface. However, if the router does not have a subnet
of the learned network number assigned to a local interface, the
router has to assume that the network is not subnetted and applies the
route’s natural classful mask.
For example, assume that Port 1 of a router has been assigned the IP
address 130.24.13.1/24 and that Port 2 has been assigned the IP address
200.14.13.2/24. If the router learns about network 130.24.36.0 from a
neighbor, it applies a /24 mask since Port 1 is configured with another
subnet of the 130.24.0.0 network. However, when the router learns
about network 131.25.0.0 from a neighbor, it assumes a “natural” /16
mask since no other masking information is available. How does a RIP-1
based router know whether it should include the subnet number bits in
a routing table update to a RIP-1 neighbor? A router executing RIP-1
will only advertise the subnet number bits on another port if the update
port is configured with a subnet of the same network number. If the
update port is configured with a different subnet or network number,
the router will only advertise the network portion of the subnet route
and zero-out the subnet number field.
For example, assume that Port 1 of a router has been assigned the IP
address 130.24.13.1/24 and that Port 2 has been assigned the IP address
200.14.13.2/24. Also, assume that the router has learned about network
130.24.36.0 from a neighbor. Since Port 1 is configured with another
subnet of the 130.24.0.0 network, the router assumes that network
130.24.36.0 has a /24 subnet mask. When it comes to advertise this
route, the router advertises 130.24.36.0 on Port 1, but it only advertises
130.24.0.0 on Port 2.
UNDERSTANDING IP ADDRESSING 20
For these reasons, RIP-1 is limited to a single subnet mask for each network
number. However, there are several advantages to be gained if
more than one subnet mask can be assigned to a given IP network number:
• Multiple subnet masks permit more efficient use of an organization’s
assigned IP address space.
• Multiple subnet masks permit route aggregation which can significantly
reduce the amount of routing information at the backbone
level within an organization’s routing domain.
Efficient Use of Assigned IP Address Space
VLSM supports more efficient use of an organization’s assigned IP
address space. The earlier limitation of supporting only a single subnet
mask across a given network prefix locked the organization into a fixed
number of fixed sized subnets.
For example, assume that a network administrator configured the
130.5.0.0/16 network with a /22 extended network prefix, as shown in
Figure 16. A /16 network with a /22 extended network prefix would
permit 64 subnets (26), each of which could support a maximum of
1,022 hosts (210-2).
Please refer to Figure 16. This configuration would be suitable if the
organization wanted to deploy a number of large subnets, but what
about the occasional small subnet containing only 20 or 30 hosts? Since
a subnetted network could have only a single mask, the network administrator
would still be required to assign the 20 or 30 hosts to a subnet
with a 22-bit prefix. This assignment would waste approximately 1,000
IP host addresses for each small subnet deployed. Limiting the association
of a network number with a single mask did not encourage the
flexible and efficient use of an organization’s address space. One solution
to this problem was to allow a subnetted network to be assigned
more than one subnet mask.
FIGURE 16. 130.5.0/16 with a /22 Extended Network Prefix
21
For example, assume that the network administrator was also allowed to
configure the 130.5.0.0/16 network with a /26 extended network prefix,
as shown in Figure 17. A /16 network address with a /26 extended network
prefix would permit 1,024 subnets (210), each of which would
support a maximum of 62 hosts (26 -2). The /26 prefix would be ideal
for small subnets with less than 60 hosts, while the /22 prefix would be
well suited for larger subnets containing up to 1,000 hosts.
Route Aggregation
VLSM also allows the recursive division of an organization’s address
space so that it can be reassembled and aggregated to reduce the
amount of routing information at the top level. Conceptually, a network
is first divided into subnets, then some of the subnets are divided into
sub-subnets, and some of the sub subnets are divided into sub-subnets.
This allows the detailed structure of routing information for one subnet
group to be hidden from routers in another subnet group.
11.0.0.0./8 11.1.0.0/16
11.2.0.0/16
11.3.0.0/16
11.252.0.0/16
11.253.0.0/16
11.254.0.0/16 11.1.1.0/24
11.1.2.0/24
11.1.253.0/24
11.1.254.0/24
11.253.32.0/19
11.253.64.0/19
11.253.160.0/19
11.253.192.0/19 11.1.253.32/27
11.1.253.64/27
11.1.253.160/27
11.1.253.192/27
FIGURE 17. 130.5.0/16 with a /26 Extended Network Prefix
In Figure 18, the 11.0.0.0/8 network is first configured with a /16
extended network prefix. The 11.1.0.0/16 subnet is then configured
with a /24 extended network prefix and the 11.253.0.0/16 subnet is
configured with a /19 extended network prefix. Note that the recursive
process does not require that the same extended network prefix be
assigned at each level of the recursion. Also, the recursive subdivision
of the organization’s address space can be carried out as far as the network
administrator needs to take it.
UNDERSTANDING IP ADDRESSING 22
FIGURE 19. Route Aggregation, Reducing Routing Table
Size
FIGURE 18. Recursive Division of a Network Prefix
23
Figure 19 illustrates how a planned and thoughtful allocation of VLSM
can reduce the size of an organization’s routing tables. Notice how
Router D can summarize the six subnets behind it into a single advertisement
(11.1.253.0/24) and how Router B can aggregate all subnets
behind it into a single advertisement (11.1.0.0/16). Likewise, Router C
can summarize the six subnets behind it into a single advertisement
(11.253.0.0/16). Finally, since the subnet structure is not visible outside
of the organization, Router A injects a single route into the global Internet’s
routing table-11.0.0.0/8 (or 11/8).
VLSM Design Considerations
When developing a VLSM design, the network designer must recursively
ask the same set of questions as for a traditional subnet design.
The same set of design decisions must be made at each level of the hierarchy:
1 How many total subnets does this level need today?
2 How many total subnets will this level need in the future?
3 How many hosts are on this level’s largest subnet today?
4 How many hosts will be on this level’s largest subnet be in the future?
At each level, the design team must ensure that they have enough extra
bits to support the required number of subentities in the next levels of
recursion.
Assume that a network is spread out over a number of sites. For example,
if an organization currently has three campuses, it probably needs 3
bits of subnetting (23 = 8) to allow the addition of more campuses in the
future. Now, within each campus, there is likely to be a secondary level
of subnetting to identify each building. Finally, within each building, a
third level of subnetting might identify each of the individual workgroups.
Following this hierarchical model, the top level is determined
by the number of campuses, the middle level is based on the number of
buildings at each site, and the lowest level is determined by the maximum
number of subnets and maximum number of users per subnet in
each building.
The deployment of a hierarchical subnetting scheme requires careful
planning. It is essential that the network designers recursively work
their way down through their addressing plan until they get to the bottom
level. At the bottom level, they must make sure that the leaf subnets
are large enough to support the required number of hosts. When
the addressing plan is deployed, the addresses from each site must be
aggregated into a single address block that keeps the backbone routing
tables from becoming too large.
UNDERSTANDING IP ADDRESSING 24
Requirements for Deploying VLSM
The successful deployment of VLSM has three prerequisites:
• The routing protocols must carry extended network prefix information
with each route advertisement.
• All routers must implement a consistent forwarding algorithm based
on the “longest match.”
• For route aggregation to occur, addresses must be assigned so that
they have topological significance.
Routing Protocols Must Carry Extended Network Prefix Lengths
Routing protocols, such as OSPF and I-IS-IS, enable the deployment of
VLSM by providing the extended network prefix length or mask value
along with each route advertisement. This permits each subnetwork to
be advertised with its corresponding prefix length or mask. If the routing
protocols did not carry prefix information, a router would have to
either assume that the locally configured prefix length should be
applied, or perform a look-up in a statically configured prefix table that
contains all of the required masking information. The first alternative
cannot guarantee that the correct prefix is applied, and static tables do
not scale since they are difficult to maintain and subject to human error.
To deploy VLSM in a complex topology, the administrator must select
OSPF or I-IS-IS as the Interior Gateway Protocol (IGP) rather than RIP-1.
Note that RIP-2, defined in RFC 1388, improves the RIP protocol by
allowing it to carry extended network prefix information. Therefore,
RIP-2 supports the deployment of VLSM.
Forwarding Algorithm Based on the Longest Match
All routers must implement a consistent forwarding algorithm based on
the longest match algorithm. The deployment of VLSM means that the
set of networks associated with extended network prefixes may manifest
a subset relationship. A route with a longer extended network prefix
describes a smaller set of destinations than the same route with a
shorter extended network prefix. As a result, a route with a longer
extended network prefix is more specific while a route with a shorter
extended network prefix is less specific. Routers must use the route
with the longest matching extended network prefix (most specific
matching route) when forwarding traffic.
25
For example, if a packet’s destination IP address was 11.1.2.5 and there
were three network prefixes in the routing table (11.1.2.0/24,
11.1.0.0/16, and 11.0.0.0/8), the router would select the route to
11.1.2.0/24. The 11.1.2.0/24 route would be selected because its prefix
has the greatest number of corresponding bits in the Destination IP
address of the packet. This concept is illustrated in Figure 20.
A very subtle but extremely important issue is that since the destination
address matches all three routes, it must be assigned to a host that
is attached to the 11.1.2.0/24 subnet. If the 11.1.2.5 address is assigned
to a host that is attached to the 11.1.0.0/16 or 11.0.0.0/8 subnet, the
routing system will never route traffic to the host since the “longest
match algorithm” assumes that the host is part of the 11.1.2.0/24 subnet.
Great care must be taken when assigning host addresses to ensure
that every host is reachable.
Topologically Significant Address Assignment
Since OSPF and I-IS-IS convey the extended network prefix information
with each route, the VLSM subnets can be scattered throughout an
organization’s topology. However, to support hierarchical routing and
reduce the size of an organization’s routing tables, addresses should be
assigned so that they are topologically significant.
Hierarchical routing requires that addresses be assigned to reflect the
actual network topology. This reduces the amount of routing information
by aggregating the set of addresses assigned to a particular region
of the topology into a single routing advertisement for the entire set.
Hierarchical routing allows this to be done recursively at various points
within the hierarchy of the routing topology. If addresses do not have a
topological significance, they cannot be aggregated and the size of the
routing tables cannot be reduced.
FIGURE 20. Best Match Route with Longest Prefix (Most
Specific)
UNDERSTANDING IP ADDRESSING 26
VLSM Example
Given
An organization has been assigned the network number 140.25.0.0/16
and it plans to deploy VLSM. Figure 21 provides a graphic display of
the VLSM design for the organization.
The first step of the subnetting process divides the base network
address into 16 equally sized address blocks. Then Subnet #1 is divided
into 32 equally sized address blocks and Subnet #14 is divided into 16
equally sized address blocks. Finally, Subnet #14-14 is divided into
eight equally sized address blocks.
Define the 16 Subnets of 140.25.0.0/16
The first step in the subnetting process divides the base network
address into 16 equally sized address blocks, as illustrated in Figure 22.
Since 16 = 24, four bits are required to identify each of the 16 subnets.
This means that the organization needs four more bits, or a /20, in the
extended network prefix to define the 16 subnets of 140.25.0.0/16.
Each of these subnets represents a contiguous block of 212 (or 4,096)
network addresses.
FIGURE 22. Sixteen Subnets for 140.25.0.0/16
FIGURE 21. Address Strategy for VLSM Example
27
The 16 subnets of the 140.25.0.0/16 address block are listed in the following
code sample. The subnets are numbered 0 through 15. The
underlined portion of each address identifies the extended network prefix,
while the bold digits identify the 4 bits representing the subnet
number field:
Base Network: 10001100.00011001 .00000000.00000000 = 140.25.0.0/16
Subnet #0: 10001100.00011001.0000 0000.00000000 = 140.25.0.0/20
Subnet #1: 10001100.00011001.0001 0000.00000000 = 140.25.16.0/20
Subnet #2: 10001100.00011001.0010 0000.00000000 = 140.25.32.0/20
Subnet #3: 10001100.00011001.0011 0000.00000000 = 140.25.48.0/20
Subnet #4: 10001100.00011001.0100 0000.00000000 = 140.25.64.0/20
:
:
Subnet #13: 10001100.00011001.1101 0000.00000000 = 140.25.208.0/20
Subnet #14: 10001100.00011001.1110 0000.00000000 = 140.25.224.0/20
Subnet #15: 10001100.00011001.1111 0000.00000000 = 140.25.240.0/20
Define the Host Addresses for Subnet #3 (140.25.48.0/20)
Figure 23 shows the host addresses that can be assigned to Subnet #3
(140.25.48.0/20).
Since the host number field of Subnet #3 contains 12 bits, there are
4,094 valid host addresses (212 -2) in the address block. The hosts are
numbered 1 through 4,094. The valid host addresses for Subnet #3 are
listed in the following sample code. The underlined portion of each
address identifies the extended network prefix, while the bold digits
identify the 12-bit host number field:
Subnet #3: 10001100.00011001.0011 0000.00000000 = 140.25.48.0/20
Host #1: 10001100.00011001.0011 0000.00000001 = 140.25.48.1/20
Host #2: 10001100.00011001.0011 0000.00000010 = 140.25.48.2/20
Host #3: 10001100.00011001.0011 0000.00000011 = 140.25.48.3/20
:
:
Host #4093: 10001100.00011001.0011 1111.11111101 = 140.25.63.253/20
Host #4094: 10001100.00011001.0011 1111.11111110 = 140.25.63.254/20
FIGURE 23. Host Address for Subnet #3 (140.25.48.0/20)
UNDERSTANDING IP ADDRESSING 28
The broadcast address for Subnet #3 is the all-1s host address or:
10001100.00011001.0011 1111.11111111 = 140.25.63.255
The broadcast address for Subnet #3 is exactly one less than the base
address for Subnet #4 (140.25.64.0).
Define the Sub-Subnets for Subnet #14 (140.25.224.0/20)
After the base network address is divided into 16 subnets, Subnet #14
is subdivided into 16 equally sized address blocks. This division is illustrated
in Figure 24.
Since 16 = 24, four more bits are required to identify each of the 16
subnets. This means that the organization will need to use a /24 as the
extended network prefix length. The 16 subnets of the 140.25.224.0/20
address block are listed in the following sample code. The subnets are
numbered 0 through 15. The underlined portion of each sub-subnet
address identifies the extended network prefix, while the bold digits
identify the 4 bits representing the sub-subnet number field:
Subnet #14: 10001100.00011001.1110 0000.00000000 = 140.25.224.0/20
Subnet #14-0: 10001100.00011001.1110 0000 .00000000 = 140.25.224.0/24
Subnet #14-1: 10001100.00011001.1110 0001 .00000000 = 140.25.225.0/24
Subnet #14-2: 10001100.00011001.1110 0010 .00000000 = 140.25.226.0/24
Subnet #14-3: 10001100.00011001.1110 0011 .00000000 = 140.25.227.0/24
Subnet #14-4: 10001100.00011001.1110 0100 .00000000 = 140.25.228.0/24
.
.
Subnet #14-14: 10001100.00011001.1110 1110 .00000000 = 140.25.238.0/24
Subnet #14-15: 10001100.00011001.1110 1111 .00000000 = 140.25.239.0/24
FIGURE 24. Sub-Subnets for Subnet #14 (140.25.224.0/20)
29
Define Host Addresses for Subnet #14-3 (140.25.227.0/24)
Figure 25 shows the host addresses that can be assigned to Subnet #14-
3 (140.25.227.0/24).
Each of the subnets of Subnet #14-3 has 8 bits in the host number field.
This means that each subnet represents a block of 254 valid host
addresses (28 -2). The hosts are numbered 1 through 254.
The valid host addresses for Subnet #14-3 are listed in the following
sample code. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 8-bit host
number field:
Subnet #14 3: 10001100.00011001.11100011 .00000000 = 140.25.227.0/24
Host #1 10001100.00011001.11100011 .00000001 = 140.25.227.1/24
Host #2 10001100.00011001.11100011 .00000010 = 140.25.227.2/24
Host #3 10001100.00011001.11100011 .00000011 = 140.25.227.3/24
Host #4 10001100.00011001.11100011 .00000100 = 140.25.227.4/24
Host #5 10001100.00011001.11100011 .00000101 = 140.25.227.5/24
.
.
Host #253 10001100.00011001.11100011 .11111101 = 140.25.227.253/24
Host #254 10001100.00011001.11100011 .11111110 = 140.25.227.254/24
The broadcast address for Subnet #14-3 is the all-1s host address or:
10001100.00011001.11100011. 11111111 = 140.25.227.255
The broadcast address for Subnet #14-3 is exactly one less than the base
address for Subnet #14-4 (140.25.228.0).
FIGURE 25. Host Addresses for Subnet #14-3
(140.25.227.0/24)
UNDERSTANDING IP ADDRESSING 30
Define the Sub-Subnets for Subnet #14-14 (140.25.238.0/24)
After Subnet #14 is divided into 16 subnets, Subnet #14-14 is subdivided
into eight equally sized address blocks, as shown in Figure 26.
Since 8 = 23, three more bits are required to identify each of the eight
subnets. This means that the organization will need to use a /27 as the
extended network prefix length.
The eight subnets of the 140.25.238.0/24 address block are listed in the
following sample code. The subnets are numbered 0 through 7. The
underlined portion of each sub-subnet address identifies the extended
network prefix, while the bold digits identify the 3 bits representing
the subnet-number field:
Subnet #14-14: 10001100.00011001.11101110 .00000000 = 140.25.238.0/24
Subnet#14-14-0: 10001100.00011001.11101110.000 00000 = 140.25.238.0/27
Subnet#14-14-1: 10001100.00011001.11101110.001 00000 = 140.25.238.32/27
Subnet#14-14-2: 10001100.00011001.11101110.010 00000 = 140.25.238.64/27
Subnet#14-14-3: 10001100.00011001.11101110.011 00000 = 140.25.238.96/27
Subnet#14-14-4: 10001100.00011001.11101110.100 00000 = 140.25.238.128/27
Subnet#14-14-5: 10001100.00011001.11101110.101 00000 = 140.25.238.160/27
Subnet#14-14-6: 10001100.00011001.11101110.110 00000 = 140.25.238.192/27
Subnet#14-14-7: 10001100.00011001.11101110.111 00000 = 140.25.238.224/27
FIGURE 26. Sub-Subnets for Subnet #14-14
(140.25.238.0/24)
31
Define Host Addresses for Subnet #14-14-2 (140.25.238.64/27)
Figure 27 shows the host addresses that can be assigned to Subnet #14-
14-2 (140.25.238.64/27).
Each of the subnets of Subnet #14-14 has 5 bits in the host number
field. This means that each subnet represents a block of 30 valid host
addresses (25 -2). The hosts will be numbered 1 through 30.
The valid host addresses for Subnet #14-14-2 are listed in the following
sample code. The underlined portion of each address identifies the
extended network prefix, while the bold digits identify the 5-bit host
number field:
Subnet#14-14-2: 10001100.00011001.11101110.010 00000 = 140.25.238.64/27
Host #1 10001100.00011001.11101110.010 00001 = 140.25.238.65/27
Host #2 10001100.00011001.11101110.010 00010 = 140.25.238.66/27
Host #3 10001100.00011001.11101110.010 00011 = 140.25.238.67/27
Host #4 10001100.00011001.11101110.010 00100 = 140.25.238.68/27
Host #5 10001100.00011001.11101110.010 00101 = 140.25.238.69/27
.
.
Host #29 10001100.00011001.11101110.010 11101 = 140.25.238.93/27
Host #30 10001100.00011001.11101110.010 11110 = 140.25.238.94/27
The broadcast address for Subnet #14-14-2 is the all-1s host address or:
10001100.00011001.11011100.010 11111 = 140.25.238.95
The broadcast address for Subnet #6-14-2 is exactly one less than the
base address for Subnet #14-14-3 (140.25.238.96).
Additional Practice with VLSM
Appendix D provides exercises for using VLSM.
FIGURE 27. Host Addresses for Subnet #14-14-2
(140.25.238.64/27)
UNDERSTANDING IP ADDRESSING 32
Classless Inter-Domain Routing (CIDR)
By 1992, the exponential growth of the Internet was raising serious concerns
among members of the IETF about the ability of the Internet’s
routing system to scale and support future growth. These problems
were related to:
• The near-term exhaustion of the Class B network address space
• The rapid growth in the size of the global Internet’s routing tables
• The eventual exhaustion of the 32-bit IPv4 address space
Throughout the Internet’s growth, the first two problems listed became
critical and the response to these immediate challenges was the development
of Classless Inter-Domain Routing (CIDR). The third problem,
which is of a more long-term nature, is currently being explored by the
IP Next Generation (IPng or IPv6) working group of the IETF.
CIDR was officially documented in September 1993 in RFC 1517, 1518,
1519, and 1520. CIDR supports two important features that benefit the
global Internet routing system:
• CIDR eliminates the traditional concept of Class A, Class B, and Class
C network addresses.
• CIDR supports route aggregation where a single routing table entry
can represent the address space of thousands of traditional classful
routes. This allows a single routing table entry to specify how to route
traffic to many individual network addresses. Route aggregation helps
control the amount of routing information in the Internet’s backbone
routers, reduces route flapping (rapid changes in route availability),
and eases the local administrative burden of updating external routing
information.
Without the rapid deployment of CIDR in 1994 and 1995, the Internet
routing tables would have in excess of 70,000 classful routes and the
Internet would probably not be functioning today.
CIDR Promotes the Efficient Allocation of the IPv4 Address Space
CIDR eliminates the traditional concept of Class A, Class B, and Class C
network addresses and replaces them with the generalized concept of a
network prefix. Routers use the network prefix, rather than the first 3
bits of the IP address, to determine the dividing point between the network
number and the host number. As a result, CIDR supports the
deployment of arbitrarily sized networks rather than the standard 8-bit,
16-bit, or 24-bit network numbers associated with classful addressing.
In the CIDR model, each piece of routing information is advertised with
a bit mask (or prefix length). The prefix length is a way of specifying
the number of leftmost contiguous bits in the network portion of each
routing table entry. For example, a network with 20 bits of network
number and 12 bits of host number would be advertised with a 20-bit
prefix length (/20). The IP address advertised with the /20 prefix could
be a former Class A, Class B, or Class C address. Routers that support
CIDR do not make assumptions based on the first three bits of the
address, they rely on the prefix length information provided with the
route.
In a classless environment, prefixes are viewed as bitwise contiguous
blocks of the IP address space. For example, all prefixes with a /20 prefix
represent the same amount of address space (212 or 4,096 host
addresses). Furthermore, a /20 prefix can be assigned to a traditional
Class A, Class B, or Class C network number. Figure 28 shows how each
of the following /20 blocks represent 4,096 host addresses-
10.23.64.0/20, 130.5.0.0/20, and 200.7.128.0/20.
Table 3 provides information about the most commonly deployed CIDR
address blocks. The table shows that a /15 allocation can also be specified
using the traditional dotted-decimal mask notation of 255.254.0.0.
Also, a /15 allocation contains a bitwise contiguous block of 128K
(131,072) IP addresses that can be classfully interpreted as two Class B
networks or 512 Class C networks.
33
FIGURE 28. Bitwise Contiguous Address Blocks
TABLE 3. CIDR Address Blocks
Host Implications for CIDR Deployment
There may be severe host implications when CIDR-based networks are
deployed. Since many hosts are classful, their user interface will not
permit them to be configured with a mask that is shorter than the natural
mask for a traditional classful address.
For example, to deploy 200.25.16.0 as a /20 to define a network capable
of supporting 4,094 (212 -2) hosts, ensure that the software executing
on each end station will allow a traditional Class C (200.25.16.0) to be
configured with a 20-bit mask since the natural mask for a Class C network
is a 24-bit mask. If the host software supports CIDR, shorter
masks can be configured.
There will be no host problems by deploying the 200.25.16.0/20 (a traditional
Class C) allocation as a block of 16 /24s since non-CIDR hosts
will interpret their local /24 as a Class C. Likewise, 130.14.0.0/16 (a traditional
Class B) could be deployed as a block of 255 /24s since the hosts
will interpret the /24s as subnets of a /16. If host software supports the
configuration of shorter than expected masks, the network manager has
tremendous flexibility in network design and address allocation.
Efficient Address Allocation
How does CIDR lead to the efficient allocation of the IPv4 address
space? In a classful environment, an Internet Service Provider (ISP) can
only allocate /8, /16, or /24 addresses. In a CIDR environment, the ISP
can carve out a block of its registered address space that specifically
meets the needs of each client, provides additional room for growth,
and does not waste a scarce resource.
Assume that an ISP has been assigned the address block 206.0.64.0/18.
This block represents 16,384 (214) IP addresses, which can be interpreted
as 64 /24s. If a client requires 800 host addresses, rather than
assigning a Class B address (and wasting approximately 64,700
addresses) or four individual Class C addresses (and introducing four
new routes into the global Internet routing tables), the ISP could assign
the client the address block 206.0.68.0/22, which is a block of 1,024
(210) IP addresses (four contiguous /24s). The efficiency of this allocation
is illustrated in Figure 29.
UNDERSTANDING IP ADDRESSING 34
FIGURE 29. CIDR Efficient Address Allocation
35
CIDR Address Allocation Example
For this example, assume that an ISP owns the address block
200.25.0.0/16. This block represents 65,536 (216) IP addresses (or 256
/24s).
The ISP wants to allocate the smaller 200.25.16.0/20 address block,
which represents 4,096 (212) IP addresses (or 16 /24s).
Address Block 11001000.00011001.00010000.00000000 200.25.16.0/20
In a classful environment, the ISP is forced to use the /20 as 16 individual
/24s.
However, in a classless environment, the ISP is free to cut up the pie
any way it wants. It could slice the original pie into pieces (each onehalf
of the address space) and assign one portion to Organization A,
then cut the other half into two pieces (each one-fourth of the address
space) and assign one piece to Organization B, and then slice the
remaining fourth into two pieces (each one-eighth of the address space)
and assign them to Organization C and Organization D. Each of the organizations
is free to allocate the address space within its “Intranetwork”
as desired. This example is illustrated in Figure 31.
FIGURE 31. Slicing the Pie-Classless Enviornment
FIGURE 30. Slicing the Pie-Classful Enviornment
The following steps explain how to assign addresses with classless interdomain
routing.
Step #1: Divide the address block 200.25.16.0/20 into two equally sized
slices. Each block represents one-half of the address space, or 2,048
(211) IP addresses.
ISP’s Block 11001000.00011001.00010000.00000000 200.25.16.0/20
Org A: 11001000.00011001.00010000.00000000 200.25.16.0/21
Reserved: 11001000.00011001.00011000.00000000 200.25.24.0/21
Step #2: Divide the reserved block (200.25.24.0/21) into two equally
sized slices. Each block represents one-fourth of the address space, or
1,024 (210) IP addresses.
Reserved 11001000.00011001.00011000.00000000 200.25.24.0/21
Org B: 11001000.00011001.00011000.00000000 200.25.24.0/22
Reserved 11001000.00011001.00011100.00000000 200.25.28.0/22
Step #3: Divide the reserved address block (200.25.28.0/22) into two
equally sized blocks. Each block represents one-eighth of the address
space, or 512 (29) IP addresses.
Reserved 11001000.00011001.00011100.00000000 200.25.28.0/22
Org C: 11001000.00011001.00011100.00000000 200.25.28.0/23
Org D: 11001000.00011001.00011110.00000000 200.25.30.0/23
Comparing CIDR to VLSM
CIDR and VLSM both allow a portion of the IP address space to be
recursively divided into subsequently smaller pieces. The difference is
that with VLSM, the recursion is performed on the address space previously
assigned to an organization and is invisible to the global Internet.
CIDR, on the other hand, permits the recursive allocation of an address
block by an Internet Registry to a high-level ISP, a mid-level ISP, a lowlevel
ISP, and a private organization’s network.
Like VLSM, the successful deployment of CIDR has three prerequisites:
• The routing protocols must carry network prefix information with
each route advertisement.
• All routers must implement a consistent forwarding algorithm based
on the longest match.
• For route aggregation to occur, addresses must be assigned so that
they are topologically significant.
Controlling the Growth of Internet’s Routing Tables
CIDR helps control the growth of the Internet’s routing tables by reducing
the amount of routing information. This process requires that the
Internet be divided into addressing domains. Within a domain, detailed
information is available about all of the networks that reside in the
domain. Outside of an addressing domain, only the common network
prefix is advertised. This allows a single routing table entry to specify a
route to many individual network addresses.
UNDERSTANDING IP ADDRESSING 36
37
Figure 32 illustrates how the allocation described in the previous CIDR
example helps reduce the size of the Internet routing tables. Assume
that a portion of the ISP’s address block (200.25.16.0/20) has been allocated
as described in the previous example:
• Organization A aggregates eight /24s into a single advertisement
(200.25.16.0/21)
• Organization B aggregates four /24s into a single advertisement
(200.25.24.0/22)
• Organization C aggregates two /24s into a single advertisement
(200.25.28.0/23)
• Organization D aggregates two /24s into a single advertisement
(200.25.30.0/23)
Then the ISP can inject the 256 /24s in its allocation into the Internet
with a single advertisement-200.25.0.0/16.
Note that route aggregation by means of BGP-4 (the protocol that allows
CIDR aggregation) is not automatic. The network engi
oh well, can't wait till tomorrow.......need tomacco!!!!!!!!!!!!!!
on this one.....@!#$ @!#$ <br> 
