Internet Addressing

Added 26 Jul 2008

Internet protocol addresses, or IP addresses, uniquely identify every network or host on the Internet. To make sure they are unique, one body, called InterNIC, is responsible for issuing them.

If your network is connected to the Internet and the computers need to be addressable from the Internet you must use IP addresses issued by InterNIC. If you don’t use InterNIC- issued addresses, you must set up the gateway between your network and the Internet so that packets containing the made- up addresses will never pass through it in either direction.

Internet addresses are 32 bits long, written as four bytes separated by periods
(full stops). They can range from 1.0. 0.1 to 223. 255. 255. 255. It’s worth noting that IP addresses are stored in big-endian format, with the most significant byte first, read left to right. This contrasts with the little-endian format used on Intel- based systems for storing 32- bit numbers. This minor point can cause a lot of trouble for PC programmers and others working with raw IP data if they forget.

IP addresses comprise two parts, the network ID and the host ID. An IP address can identify a network (if the host part is all zero) or an individual host. The dividing line between the network ID and the host ID is not constant. Instead, IP addresses are split into three classes which allow for a small number of very large networks, a medium number of medium- sized networks and a large number of small networks.

Class A addresses have a first byte in the range 1 to 126. The remaining three bytes can be used for unique host addresses. This allows for 126 networks each with up to 16m hosts.

Class B addresses can be distinguished by first byte values in the range 128. 0. x. x to 191. 255. x. x. In these addresses, the first two bytes are used for the net ID, and the last two for the host ID, giving addresses for 16,000 networks, each with up to 16,000 hosts.

Class C addresses are in the range 224.0. 0.x to 239. 255. 255. x. Here, the first three bytes identify the network, leaving just one byte for the individual hosts. This provides for 2 million networks of up to 254 hosts each.

Although these addresses make it possible to uniquely identify quite a lot of networks and hosts, the number is not that large in relation to the current rate of expansion of the Internet. Consequently, a new addressing system has been devised which is part of Internet Protocol version 6 (IPv6). IPv6 won’t come into use for a couple of years, and understanding it isn’t essential to understanding how IP works in general, so we won’t cover it here.

IP addresses can be further divided to obtain a subnet ID. The main net ID identifies a network of networks. The subnet ID lets you address a specific network within that network. This system of addressing more accurately reflects how real- world large networks are connected together.

You decide how the subnet ID is arrived at by defining a 32- bit value called the subnet mask. This is logically ANDed with the IP address to obtain the subnet address. For example, if a subnet mask was 255.255. 255.0 and an IP address was 128. 124. 14.5, 128. 124 would identify the Class B network, 128. 124. 14 would identify the subnetwork, and 5 would identify the host on that subnetwork.