IP
Addressing and Allocation Techniques
This chapter covers
the following key topics:
Overview
of Internet addressing
Provides an overview
of IPv4 Class A/B/C addressing and basic subnetting concepts.
Variable-length
subnet masks (VLSMs)
Provides a description
of variable-length subnet masks and how they can be used in efficient
assignment of the IP address space.
IP
address space depletion
Discusses how the
problem of IP address space depletion is being managed through creative address
allocation, supernetting, private addressing, and next-generation protocols.
Private
addressing and Network Address Translation (NAT)
Discusses how NAT
software is used to map between private and global IP addresses.
IP
version 6 (IPv6)
Provides an overview
of the IP next generation (IPng) addressing scheme and how it maps to the
hierarchical model offered by CIDR and IPv4.
This
chapter begins with a brief history of Internet addressing, providing
information on traditional IP version 4 (IPv4) addressing and subnetting
models. From there, you'll learn about some of the issues surrounding address
space depletion on the Internet. Then we'll examine IP addressing techniques
and allocation strategies such as variable-length subnet masking (VLSM),
classless interdomain routing (CIDR), and Network Address Translation (NAT).
The chapter finishes with a brief introduction to IP version 6 (IPv6).
Addressing
strategies are of direct and fundamental relevance to routing architecture in
any network. One of the basic functions of routing architecture and routers is
to accommodate addresses for all the traffic they direct. With the explosive
growth of the Internet, the sheer number of addresses and the evolution of new
addressing strategies have presented new challenges for routing architectures.
An understanding of the history and fundamentals of IP addressing will no doubt
play a key role in your ability to quickly grasp routing protocol concepts.
History
of Internet Addressing
The
addressing scheme that is used today in the Internet is based on version 4 of
the Internet Protocol (IPv4), usually referred to simply as IP. This
section discusses the following:
- Basic IP addressing
- Basic IP subnetting
- Variable-length subnet mask (VLSM)
Basic
IP Addressing
An IP address is a unique
4-octet (32-bit) value expressed in dotted-decimal (or dotted-quad) notation of
the form W.X.Y.Z, where periods (dots) are used to separate each of the 4
octets of the address (for example, 10.0.0.1). The 32-bit address field
consists of two parts: a network or
link number (which represents the network portion of the address) and a host
number (which identifies a host on the network segment). The network and
host boundaries were traditionally defined based on the class of the IP address,
with five defined classes (three of which are used for unicast addressing): A,
B, C, D, and E. Table 3-1 illustrates the different classes of address space and their
functions. Table 3-1, IP Address Classes and Functions
Class
Address Range High-Order Bits Network Bits Host Bits Function
Notice that only
Class A, B, and C addresses are used for unicast. Class D addresses are used
for
multicast, and Class E address space is reserved. Several addresses within
these classes are
reserved for special
use. Table 3-2 lists some of these addresses.
This class-based
addressing scheme is often referred to as the classful model. The
different
classes lend
themselves to different network configurations, depending on the desired ratio
of
networks to hosts.
The full implications of the different classes will become more apparent as
this chapter
proceeds. The next few sections focus on the basic definitions of each class.
Class
A Addressing
Class A networks are
represented by a 0 in the leftmost bit position of the address. The first
octet (bits 0 to 7)
of the address, beginning from the leftmost bit, represents the network
number, and the
remaining 3 octets (bits 8 to 31) represent a host number on that network. An
example of a Class A
network is 124.0.0.1, where 124.0.0.0 represents the network number
and the host number
is 1. The outcome of this representation, illustrated in Figure
3-1, is
128(27) Class A
network numbers. However, because 0.0.0.0 is not a valid network number,
only 127(27–1) Class
A addresses are possible.
Figure
3-1. General Class A Address Format
After the network is
defined, the first and last host addresses within the network serve special
functions. The first
address (124.0.0.0 in the previous example) is used to represent the
network number, and
the last address of the network is used to represent the directed
broadcast
address of the network (124.255.255.255). Therefore, Class A addresses have
only
16,777,214 (224–2)
hosts per network, rather than 16,777,216 (224) hosts per network.
Class
B Addressing
Class B networks are
represented by a 1 and a 0 in the leftmost two bits of the address. The
first two octets of
the address (bits 0 to 15) represent the network portion of the address, and
the
remaining two octets (bits 16 to 31) represent the host number of that network.
The
outcome of this
representation, illustrated in Figure 3-2, is
16,384 (214) network numbers,
with 65,534 (216–2)
hosts per network. An example of a Class B address is 172.16.0.1, where
172.16.0.0 is the
Class B network and 1 is the host.
Figure
3-2. General Class B Address Format
Class
C Addressing
Class C networks are
represented by 1, 1, and 0 in the leftmost three bits of the address. The
first three octets
(bits 0 to 23) represent the network number, and the last octet (bits 24 to 31)
represents the host
number in that network. The outcome of this representation, as illustrated
in Figure
3-3, is 2,097,152 (221) network numbers with 254 (28–2) hosts per
network. An
example of a Class C
network is 192.11.1.1, where 192.11.1.0 is the network number and the
host number is 1.
Figure
3-3. General Class C Address Format
Class
D Addressing
Class D networks are
represented by 1, 1, 1, and 0 in the leftmost 4 bits of the address. The
Class D address space
is reserved for multicast, used to represent multicast group numbers.
Class
E Addressing
Class
E networks are represented by 1, 1, 1, and 1 in the leftmost 4 bits of the
address. Class E
address space is
currently reserved for experimental use.
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