Subnet mask

Subnet addressing is an extension of the IP address scheme that lets a site use a single net ID for multiple physical networks. Routing outside of the site continues by dividing the IP address into a net ID and a host ID via the class. Inside a site, the subnet mask is used to divide the IP address into a custom net ID portion and host ID portion. This field is set to 0.0.0.0 by default.

For example, using Network 2, a Class B network, and add another physical network. Selecting this subnet mask adds two additional net ID bits, allowing for four physical networks:

11111111 11111111 11000000 00000000 = 255.255.192.0

Two bits of the Class B host ID have been used to extend the net ID. Each unique combination of bits in the part of the host ID where subnet mask bits are 1 specifies a different physical network.

The new configuration is:

A second network with Hosts D and E has been added. Gateway G2 connects Network 2.1 with Network 2.2. Hosts D and E will use Gateway G2 to communicate with hosts not on Network 2.2. Hosts B and C will use Gateway G to communicate with hosts not on Network 2.1. When B is communicating with D, G (the configured Gateway for B) will route the data from B to D through G2.