A network ID refers to a part of a TCP/IP address that is used to identify the subnet that a host may be on. The subnet that the computer is on is determined by the netmask and IP address of the computer. This subnet address is the same as the network ID and is the beginning part of the computers IP address.
When the netmask is setup, it is a number where some of the most significant bits have a 1's value and the rest have values of 0. The most significant part of the netmask with bits set to 1's specifies the network address, and the lower part of the address will specify the host address.
The part of the IP address that matches the part of the netmask where the bits are set to ones determines the network ID.