Format of Internet Protocol (IP) addresses

Format of Internet Protocol (IP) addresses

An IP address is a 32-bit number that network software uses to identify a system on a network. For the sake of human readability, these addresses are usually represented as four one-byte integers, separated by dots (for example, 150.166.248.17). Every system on an IP network must have its own unique IP address for the network to function properly. Systems with more than one network interface must have a unique IP address for each interface.

Note: Unlike a system's Ethernet address, a system's IP address is determined by the network and network system administrators. Conceptually, each 32-bit IP address is a pair of numbers where one number represents the network and the other the system itself. There are four classes of addresses in use (A through D). The class of address is determined by the first bits of the address:

Class A addresses begin with 0 and have 7 bits for the network number and 24 bits for the host number.
Class B addresses begin with 10 and have 14 bits for the network number and 16 bits for the host number.
Class C addresses begin with 110 and have 21 bits for the network number and 8 bits for the host number.
Class D addresses begin with 1110 and are special "multicast" addresses for use within a network site.

In all cases, host numbers 0 and 255 are reserved, and may not be used for actual systems.

Figure 2-2 shows the format of the different classes of Internet addresses.

Figure 2-2 : The Format of Internet Protocol (IP) Addresses To simplify Internet addressing, dotted decimal notation is used to break the 32-bit number into four decimal numbers separated by dots.

For example, the IP address 128.74.41.123 in binary is:

10000000| 01001010| 00101001| 01111011

128 | 74 | 41 | 123

Class A, B and C IP addresses in dot notation conform to the following specifications:

Class A -- 001.hhh.hhh.hhh through 126.hhh.hhh.hhh

Class B -- 128.001.hhh.hhh through 191.254.hhh.hhh

Class C -- 192.000.001.hhh through 223.255.254.hhh

Note: hhh is the local system and the leading numbers are the network. Networks are usually identified by network numbers--IP addresses in which the host portion is not specified. For example, 150.166 represents a Class B network, and 192.26.80 represents a Class C network.

If your network will be connected to the Internet, then you must obtain a unique network number, as described in "Obtaining a Network Number". All the systems on your network must have IP addresses allocated from your network.

If you are adding a machine to an existing network, its IP address must be allocated from that network.