|  | 10.6 | Basics of Subnetting
| 10.6.4 | Subnet mask
The subnet mask (formal term: extended network prefix), tells the network devices which part of an address is the network field and which part is the host field. A subnet mask is 32 bits long and has 4 octets, just like an IP address. To determine the subnet mask for a particular subnetwork IP address follow these steps. (1) Express the subnetwork IP address in binary form. (2) Replace the network and subnet portion of the address with all 1s. (3) Replace the host portion of the address with all 0s. (4) As the last step convert the binary expression back to dotted-decimal notation. Note: The extended network prefix includes the class A, B, or C network number, plus the subnet field (or subnet number) that is being used to extend the routing information (which is otherwise just the network number).