IP Subnet Masks - Transwiki. IP mask numbers are used to divide internet addresses into blocks called subnets.
The mask number represents the number of 1s in the binary of the address that is 'masked" against the address so that it ignores the last bits which are for the group of IP addresses in the masked address. The first address of a subnet block (all 0s) is called the network address or network ID.
The last address (all 1s) is the broadcast address of the network. CCNA Lab Workbook Table of Contents.