IPv4 Address Formats

IP was standardized in September 1981. Its address architecture was as forward-looking as could be expected, given the state of computing at that time. The basic IP address was a 32-bit binary number that was compartmentalized into four 8-bit binary numbers, or octets.

To facilitate human usage, IP’s machine-friendly binary addresses were converted into a more familiar number system: base 10. Each of the four octets in the IP address is represented by a decimal number, from 0 to 255, and is separated by dots (.). This is known as a dotted-decimal format. Thus the lowest possible value that can be represented within the framework of an IPv4 address is 0.0.0.0, and the highest possible value is 255.255.255.255. Both of these values, however, are reserved and cannot be assigned to individual end systems. The reason for this requires an examination of the way that the IETF implemented this basic address structure in its protocol.

The dotted-decimal IPv4 address was then broken down into classes, to accommodate large, medium, and small networks. The differences between the classes were the number of bits allocated to network versus host addresses. The five classes of IP addresses, identified by a single alphabetic character, are

• Class A
• Class B
• Class C
• Class D
• Class E

Each address consists of two parts: a network address and a host address. The five classes represent different compromises between the number of supportable networks and hosts.