Subnets and Subnet Masks

If you assume that an IP address identifies a computer, it's fair to say that a subnet identifies where the computer is. The component of an IP address that determines whether packets are to be sent locally or through a router is the subnet mask. The subnet mask tells the protocol whether the local host and the destination host are on the same subnet.

A subnet mask effectively breaks a complete IP address into two parts: the network identifier and the host identifier. Think of this in relationship to your home street address, which also has two parts: the street name (the network identifier) and the house number (the host identifier).

Basically, when the subnet bits match, the host and destination host are on the same subnet, and no routing is needed. When the subnet bits don't match, the destination host is on a remote network, and the packets are sent to the router.

The Internet community has standardized default subnet masks for each of the three IP address classes. The subnet masks for these address classes are defined in Table 1.4.

Table 1.4. Default IP Class Subnet Masks

Address Class	Bits for Subnet Mask	Subnet Mask
A	11111111 00000000 00000000 00000000	255.0.0.0
B	11111111 11111111 00000000 00000000	255.255.0.0
C	11111111 11111111 11111111 00000000	255.255.255.0

If a computer has an IP address of 142.146.102.45 with a subnet mask of 255.255.0.0, the first two octets (142.146) identify the network, and the last two octets (102.45) identify a specific host on that network.