IP Addressing

Each TCP/IP host is identified by a logical IP address . The IP address is a network layer address and has no dependence on the Data-Link layer address (such as a MAC address of a network adapter). A unique IP address is required for each host and network component that communicates using TCP/IP.

The IP address identifies a system's location on the network in the same way a street address identifies a house on a city block. Just as a street address must identify a unique residence, an IP address must be globally unique and have a uniform format.

Each IP address includes a network ID and a host ID.

  • The network ID (also known as a network address ) identifies the systems that are located on the same physical network bounded by IP routers. All systems on the same physical network must have the same network ID. The network ID must be unique to the internetwork.

  • The host ID (also known as a host address) identifies a workstation, server, router, or other TCP/IP host within a network. The address for each host must be unique to the network ID.


Network ID refers to any IP network ID, whether it is class-based, a subnet, or a supernet.

An IP address consists of 32 bits. Rather than working with 32 bits at a time, it is a common practice to segment the 32 bits of an IP address into four 8-bit fields called octets . Each octet is converted to a decimal number (the Base 10 numbering system) in the range 0-255 and separated by a period (a dot). This format is called dotted decimal notation. Table 1.10 provides an example of an IP address in binary and dotted decimal formats.

Table 1.10 An IP Address in Binary and Dotted Decimal Formats

Binary Format

Dotted Decimal Notation

11000000 10101000 00000011 00011000

The notation w.x.y.z is used when referring to a generalized IP address, and is shown in Figure 1.3.


Figure 1.3 IP Address