IP Address - Network Numbering

ven without subnetting (explained later), hosts on the Internet or any other IP network are assigned a network number. Network numbering allows a group of hosts (peers) to communicate efficiently with each other. Hosts on the same network may be computers located in the same facility or all computers used by a workgroup, for example. Multi-homed hosts, that contain multiple network adapters, can belong to multiple networks, but each adapter is assigned exactly one network number.

Network numbers look very much like IP addresses, but the two should not be confused. Consider for example the host IP address 10.0.0.1, an address commonly used on private networks. Because it is a Class A address, with no subnetting employed, its leftmost byte (eight bits) by default refer to the network address and all other bits remain set at zero. Thus, 10.0.0.0 is the network number corresponding to IP address 10.0.0.1.

The portion of the IP address that does not refer to the network refers instead to the host address - literally, the unique identifier of the host on that network. In the above example, the host address becomes '0.0.0.1' or simply '1'. Also note that a network address becomes a reserved address that should not be assigned to any actual host. Configuring a live host at 10.0.0.0 in the example above could impact communications for all hosts on that network.

The table below illustrates the default numbering scheme for Class A, B, and C networks.

• Clas A | 0.0.0.0 - 127.255.255.255 | x.0.0.0 | 255.0.0.0
• Clas B | 128.0.0.0 - 191.255.255.255 | x.x.0.0 | 255.255.0.0
• Clas C | 192.0.0.0 - 223.255.255.255 | x.x.x.0 | 255.255.255.0

In general, a network address uses the leftmost byte of its hosts' addressing if the hosts fall within the Class A range, the leftmost two bytes for hosts in Class B, and the leftmost three bytes for hosts in Class C. This algorithm is applied in practice through the use of a network mask. The above table shows the decimal representation of the default network masks that is commonly used by network operating systems. Note that the decimal value '255' corresponds to one byte that has all bits set to one (11111111).