Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 39
IP Address Format

Each host on a TCP/IP network is uniquely identified at the IP layer with an address that takes the form of <netid, hostid>. The address is not really separated and is read as a whole. The whole address is always used to fully identify a host. There is no separation between the fields. In fact, when an IP address is written, it is hard to tell the distinction between the two fields without knowing how to separate them.

The following shows the generalized format of an IP address:

<Network Number, Host Number> in the form of xxx.xxx.xxx.xxx

In decimal, the address range is 0.0.0.0 through 255.255.255.255. 128.4.70.9 is an example of an IP address. When looking at this address, it is hard to tell which is the network number and which is the host number, let alone a subnet number. Except for the first byte, any of the bytes can indicate a network number or host number. The first byte always indicates a network number. In order to understand how this is accomplished, let’s look first at how IP addresses are divided.

Each byte (or in Internet terms, an octet) is 8 bits long, naturally! Each of the bytes, however, can identify a network, a subnetwork, or a host.

As shown in the slide, there are 32 bits separated into 4 bytes that are used to represent an IP address. The network number can shift from the first byte to the second byte to the third byte. The same can happen to the host portion of the address. xxx represents a decimal number from 0 to 255 (the reason for three xs).

IP addresses are divided into five classes: A, B, C, D, and E. RFC 791, which classified these types, did so without the foregoing knowledge of subnets. The classes allowed for various amounts of networks and hosts to be assigned. Classes A, B, and C are used to represent host and network addresses. Class D is a special type of address used for multicasting (for example, OSPF routing updates use this type of address as well as IP multicast). Class E is reserved for experimental use.

For those trying to figure out this addressing scheme, it is best if you also know the binary numbering system and are able to convert between decimal and binary. Finally, IP addresses are sometimes expressed in hexadecimal and it is helpful to know. IPv6 uses only hexadecimal. The most common form for IPv4 is decimal. This book shows most addresses in binary and decimal.