The portion of an IP address that identifies the TCP/IP network on which a host resides. The network ID portion of an IP address uniquely identifies the host’s network on an internetwork, while the host ID portion of the IP address identifies the host within its network. Together, the host ID and network ID, which make up the entire IP address of a host, uniquely identify the host on a TCP/IP internetwork.
The network ID is found by logically ANDing the binary form of the IP address with the binary form of the subnet mask for the network. For example, if a host has an IP address of 172.16.8.55 on a network with a subnet mask of 255.255.0.0 (the default subnet mask), the network ID of the host is 172.16.0.0, or simply 172.16.
When you assign network IDs to local area networks (LANs) connected by routers, each LAN and each WAN link must have a unique network ID.