TCP/IP model has three different levels of addressing:
- Physical (Link) address
- Internetwork (IP) address
- Transport or Port address
Physical address
This is the address of a node at the datalink layer as defined by the LAN OR WAN .It is included in the frame sent by the datalink layer. This address determines the host system on a particular network. The size and format of the physical address is not defined by the TCP/IP and it depends on the kind of the network. For example, Ethernet LAN uses 6 byte (48 bit) physical address which is imprinted into the Network Interface Card.
Physical address can be either unicast (single recipient), multicast (a group of recipients) or broadcast (received by all in the network). However not all networks supports these. Ethernet – one of the popular LAN –supports all of these.
Internet address
Internet address are necessary for universal communication services that are independent of underlying physical network. Physical networks have different addressing format depending upon the network technology used. Also the addressing doesn’t have any component using which one can identify the network to which it is connected ; which is essential for the routing purpose. The Internet addresses are designed for this purpose. An Internet address is currently a 32 bit (4 byte) address which can uniquely identify a host connected to the Internet. No two hosts on the Internet can have the same IP ADDRESS. Also Internet addresses are defined such a way that given an IP address one can easily identify the network to which it is connected so that routing becomes easy.
The Internet addresses support unicast , multicast and broadcast addressing.
Port address
The IP address and the physical address identify the source and the destination systems. They don’t identify the process (a running programme on the computer) on these sytems to which the data actually correspond to. The final objective of the Internet communication is providing a communication link between two processes running on two different systems. For example, data sent by the FTP (File Transfer Protocol) client process from system . A should reach the FTP server process at the system B. It should not reach the MAIL server process running on the system B. So it is not only crucial to identify the end systems to which the data meant, but the end processes are also to be identified. To achieve this, different processes are labeled uniquely. In TCP/IP this labeling is called as port address. A port address is 16 bits long (2 byte).