Technical Terms and Definitions

Subnet Mask - Networking

To facilitate intra-network routing, a single IP network can be divided into many subnets by using some of the most significant bits of the host address portion of the IP address as a subnet ID.

For example, Network has 16 bits assigned as the network ID (specifically 129.5, in dotted decimal, which is 10000001.00000101 in binary) as it is a Class B address (the first number is between 128 and 192).

This leaves the lower 16 bits as the host address.

Using a Subnet Bit Mask of (which in binary is 11111111.11111111.11111111.00000000), specifies that the upper 24 bits (those that are set to a 1) are the network plus subnet address--that is, the 16 bits that we already expected plus the upper 8 bits of the host address. Therefore the network (in this example) would consist of up to 254 subnets ( through of up to 254 hosts each.

This is useful for subdividing networks, for example, to reduce the number of stations that must receive broadcasts.