Introduction

You've heard it said: "We're running out of IP addresses!" Really? In the IP (version 4) architecture, we use 32-bit address fields. With 32-bits in our addresses, there are 2 ³² unique addresses available. That's over four billion addresses! We know that the Internet has experienced exponential growth over the last few years, but even with continued growth, it's unlikely that we'll see anywhere near four billion machines on the Internet any time soon.

So where's the problem? The problem exists in the granularity of address allocation. Prior to Classless Inter-Domain Routing (CIDR), addresses were allocated in classful blocks. That is, if you needed more addresses than a class C network provided, you got a class B network address; if you needed more than a class B provided, you got a class A network address. Those were the only three choices. (Not many organizations actually got class A addresses, of course.)

Although there are indeed over 4 billion unique IP addresses available with the current version of IP, the number of unique network numbers is much less. In fact, there are only 126 class A networks, about 16,000 class B networks, and about 2 million class C networks. This design has led to widespread waste of globally-unique IP addresses.

Strategies to Conserve Addresses

In the 1970s, the architects of the Internet envisioned an internetwork with dozens of networks and hundreds of nodes. They developed a design where any node on the internetwork was reachable by any...