Network Addressing

In an IP¹⁾network, the address is a 32-bit number, normally written as four 8bit numbers expressed in decimal form, separated by periods. Examples of IP addresses are 10.0.17.1, 192.168.1.1, or 172.16.5.23.

Interconnected networks must agree on an IP addressing plan. In the global Internet, committees of people allocate groups of IP addresses with a consistent, coherent method to ensure that duplicate addresses are not used by different networks and so that a shorthand can be used to refer to groups of addresses. These groups of addresses are called sub-networks, or subnets for short. Larger subnets can be further subdivided into smaller subnets. Sometimes a group of related addresses is referred to as an address space.

On the Internet, no person or organization really owns these groups of addresses because the addresses only have meaning if the rest of the Internet community agrees with their usage. By agreement, the addresses are allocated to organizations according to their need and size. An organization which has been allocated an address range may then allocate a portion of that address range to another organization as part of a service agreement. Addresses which have been allocated in this manner, starting with internationally recognized committees, and then broken down hierarchically by national or smaller regional committees are referred to as globally routed IP addresses.

Sometimes it is inconvenient or impossible to get more than one globally routed IP address allocated to an individual or organization. In this case a technique knows as Network Address Translation, or NAT canbeused. A NAT device is a router with two network ports. The outside port uses one globally routed IP address, while the inside port uses an IP address from a special range known as private addresses²⁾. The NAT router allows the single global address to be shared with all of the inside users, who all use private addresses. It converts the packets from one form of addressing to the other as the packets pass through it. As far as the network users can tell, they are directly connected to the Internet and require no special software or drivers to share the single globally routed IP address.

1	In this book we deal primarily with IPv4, the version of the Internet Protocol in most common use today. While IPv6 will likely replace IPv4 at some point in the future, discussion of IPv6 is currently outside the scope of this book.
2	Private addresses are defined in RFC 1918