The portion of an IP address that uniquely identifies a host on a given TCP/IP network. You find the host ID by logically NANDing the binary form of the IP address with the binary form of the subnet mask for the network. The other part of an IP address is the network ID, which specifies the network to which the host belongs.
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 host ID is 0.0.8.55 or simply 8.55. The host ID uniquely identifies the host, but only within the boundary of the network ID for the network on which the host is installed.