An IP address is a network layer (Layer 3) address that uniquely identifies a host, including network
components and devices, on a TCP/IP network. An IP address is composed of 32 binary bits and consists of
two parts: a network ID and a host ID.
• The Network ID identifies the TCP/IP hosts that are located on the same physical network. All hosts on
the same physical network must be assigned the same network ID to communicate with each other. If
routers connect your networks, a unique network ID is required for each wide area connection.
• The Host ID identifies the individual hosts within a network. The host ID must be unique to the network
designated by the network ID.
The boundary between the network ID and the host ID of the IP address is defined by the subnet mask,
which is another 32-bit field. There is a bit-for-bit alignment between the IP address and the subnet mask.
The subnet mask contains a continuous field of 1s followed by a continuous field of 0s. The contiguous 1s
stop at the boundary between the network ID and the host ID of the IP address. The network boundary can
occur at any place after the eighth bit position from the left. Once the boundary between the network part
and the host part of the IP address is known, all devices addressed in that network will have a common
binary pattern in the network part that identifies the device as belonging to the specified network.
There are a number of formats for referencing an IP address. These include binary, dotted decimal notation
and Classless Interdomain Routing (CIDR) Notation.