This chapter covers the following key topics:

•  Overview of IPv4 Addressing
An overview of the IPv4 class A/B/C addressing and basic subnetting concepts.

•  Variable Length Subnet Mask (VLSM)
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
Through creative address allocation, supernetting, private addressing, and next generation protocols, the problem of IP address space depletion is being managed.

•  Classless Interdomain Routing (CIDR)
CIDR, one of the major key elements of the Border Gateway Protocol 4 (BGP4), is used to control the growth of the global IP routing tables. Coverage includes a detailed description of different provider/customer addressing schemes in single and multihomed environments and tips about route aggregation.

•  Private Addressing and Network Address Translation (NAT)
Network address translation software is used to map between private and global IP addresses.

•  IP Version 6 (IPv6)
An overview of the IP next generation (IPng) addressing scheme and how it maps to the hierarchical model offered by CIDR and IPv4.

Chapter 3
Handling IP Address Depletion

The overall model of address assignment continues to evolve. One of the major problems facing the Internet community is the depletion of IP addresses; this mandates the implementation of new IP addressing strategies. This chapter summarizes these strategies, their relative merits, and the issues surrounding address assignment on the Internet.

Addressing strategies are of direct and fundamental relevance to routing architecture. One of the basic functions of routing architecture and routers is to accommodate addresses for all the traffic that 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 architecture. Throughout this chapter, we will note particular routing rules and issues as they pertain to IP addressing.

This chapter begins with an overview of the basic IP addressing and subnetting models.

Overview of IPv4 Addressing

The IP addressing scheme that is widely used today is relevant to the IP version 4 (IPv4) implementation. This section discusses the following:

•  Basic Addressing [1]

•  Basic Subnetting [2]

•  Variable Length Subnet Mask (VLSM) [3]

Basic Addressing

The IP address, a 32-bit address, is represented by a dotted decimal notation of the form X.Y.Z.T (for example, 10.0.0.1). The 32-bit address field consists of two parts: a network number and a host number whose boundaries are defined based on the class of the IP address. The different IP classes are: A, B, and C¹. This addressing scheme is sometimes referred to as the classfull 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. For now, the chapter focuses on basic definitions of each class.

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¹Classes D and E for multicast and reserved addresses are beyond the scope of this book.

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Class A Addressing

A class A network is represented by a 0 in the first bit. The first 8 bits (0-7) represent the network number, and the remaining bits (8-31) represent a host number on that network. The outcome of this representation, indicated in Figure 3-1, is 128 (2⁷) class A network numbers having 16777216 (2²⁴) hosts per network (ignoring the boundaries such as all 0s and all 1s hosts that have special meaning). An example of a class A network is 10.0.0.1, representing network 10.0.0.0 and host 1.

Figure 3-1  General class A address format.

Class B Addressing

A class B network is represented by a 1 and a 0 in the first two bits. The first 16 bits (0-15) represent the network number, and the last 16 bits (16-31) represent the host number on that network. The outcome of this representation, indicated in Figure 3-2, is 16384 (2¹⁴) network numbers with 655366 (2¹⁶) hosts per network (also ignoring boundaries). An example of a class B network 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

A class C network is represented by a 1 and a 1 and a 0 in the first three bits. The first 24 bits (0-23) represent the network number, and the last 8 bits (24-31) represent the host number on that network. The outcome of this representation, indicated in Figure 3-3, is 2097152 (2²¹) network numbers with 256 (2⁸) hosts per network (ignoring boundaries). An example of a class C network is 192.11.1.1, where 192.11.1.0 is the class C network and 1 is the host.

Figure 3-3  General class C address format.