Addresses belonging to three different layers of TCP/IP architecture are describe below:

• Physical (local or link) address

At the physical level, the hosts and routers are recognised by their physical addresses. The physical address is the lowest-level address which is specified as the node or local address defined by LAN or WAN. This local address is included in the frame used by the network access layer.

A local address is called a physical address because it is usually (but not always) implemented in hardware. Ethernet or token ring uses a six-byte address that is imprinted on the network interface card (NIC) installed in the host or router.

The physical address should be unique locally, but not necessary universally. Physical addresses can be either unicast (one single recipient), multicast (a group of recipients), or broadcast (all recipients on the network). The physical addresses will be changed as a packet moves from network to network.

• IP address

An IP address is called a logical address at the network level because it is usually implemented in software. A logical address identifies a host or router at the network level. TCP/IP calls this logical address an IP address. Internet addresses can be either unicast, multicast or broadcast.

IP addresses are essentially needed for universal communication services that are independent of underlying physical networks. IP addresses are designed for a universal addressing system in which each host can be identified uniquely. An Internet address is currently a 32-bit address which can uniquely define a host connected to the Internet.

• Port address

The data sequences need the IP address and the physical address to move data from a source to the destination host. In fact, delivery of a packet to a host or router requires two levels of addresses, logical and physical. Computers are devices that can run multiple processes at the same time.

For example, computer A communicates with computer B using TELNET. At the same time, computer A can communicate with computer C using File Transfer Protocol (FTP). If these processes occur simultaneously, we need a method to label different processes.

In TCP/IP architecture, the label assigned to a process is called a port address. A port address in TCP/IP is 16 bits long. The Internet Assigned Numbers Authority (IANA) manages the well-known port numbers between 1 and 1023 for TCP/IP services.

Ports between 256 and 1023 were normally used by UNIX systems for UNIX-specific services, but are probably not found on other operating systems. Servers are normally known by their port number.

For few examples, every TCP/IP implementation that provides a File Transfer Protocol (FTP) server provides that service on TCP port 21. Telnet is a TCP/IP standard with a port number of 23 and can be implemented on almost any operating system. Hence, every Telnet server is on TCP port 23.

Every implementation of the Trivial File Transfer Protocol (TFTP) is on UDP port 69. The port number for the Domain Name System is on TCP port 53.