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Masks
The portion of the IP address to use as the network address is specified by using a mask; a
mask is the contiguous number of bits to be used for the network address all set to 1. When
the mask is logically ANDed with an IP address, the result is the network address. The mask
is specified by entering the mask size as the number of bits in the mask. For the standard
Class A, B and C Internet addresses, the mask sizes would be 8, 16 and 24 respectively.
Networks are not restricted to the above standard sizes; the mask (and hence the network
address it specifies) may be any number of bits from 8 to 32. This gives much more
flexibility to match the size of the two fields of the IP address to the number of networks
and hosts to be serviced.
IP Subnets
An IP network may be divided into smaller networks by a process called sub-netting. A
subnet is specified using some of the high order bits of the host field of the IP address for
sub-network addressing. The portion of the IP address to be used as the subnet address is
defined by using a subnet mask.
If the company in the example above (Class C IP address 199.169.100.0) decides to split
their network into two LANs to reduce the load on their network, the original IP network
address may be sub-netted into two or more smaller IP networks consisting of a smaller
number of host addresses in each LAN. This allows each of the sites to be a smaller IP
network and to be routed together to allow inter-network communication.
The subnet mask size is the number of bits in the subnet mask. In the above figure the subnet
mask size would be 26 (24 bits for the class C network address and 2 subnet bits). The subnet size
is the number of subnet bits - in the above figure, the subnet size would be 2.