An addressing scheme is clearly a requirement for communications in a computer network. With an addressing scheme, packets are forwarded from one location to another. Each of the three layers, 2, 3, and 4, of the TCP/IP protocol stack model produces a header, as indicated in Figure 1.12. In this figure, host 1 communicates with host 2 through a network of seven nodes, R1 through R7, and a payload of data encapsulated in a frame by the link layer header, the network layer header, and the transport layer header is carried over a link. Within any of these three headers, each source or destination is assigned an address as identification for the corresponding protocol layer. The three types of addresses are summarized as follows.
Link layer (layer 2) address. A 6-byte (48-bit) field called Media Access Control (MAC) address that is represented by a 6-field hexadecimal number, such as 89-A1-33-2B-C3-84, in which each field is two bytes long. Every input or output of a networking device has an interface to its connected link, and every interface has a unique MAC address. A MAC address is known only locally at the link level. Normally, it is safe to assume that no two interfaces share the same MAC address. A link layer header contains both MAC addresses of a source interface and a destination interface, as seen in the figure.
Network layer (layer 3) address. A 4-byte (32-bit) field called Internet Protocol (IP) address that is represented by a 4-field dot-separated number, such as 192.2.32.83, in which each field is one byte long. Every entity in a network must have an IP address in order to be identified in a communication. An IP address can be known globally at the network level. A network layer header contains both IP addresses of a source node and a destination node, as seen in the figure.
Transport layer (layer 4) address. A 2-byte (16-bit) field called port number that is represented by a 16-bit number, such as 4,892. The port numbers identify the two end hosts’ ports in a communication. Any host can be running several network applications at a time and thus each application needs to be identified by another host communicating to a targeted application. For example, source host 1 in Figure 1.12 requires a port number for communication to uniquely identify an application process running on the destination host 2. A transport layer header contains the port numbers of a source host and a destination host, as seen in the figure. Note that a transport-layer “port” is a logical port and not an actual or a physical one, and it serves as the end-point application identification in a host.
The details of the link layer header, including the MAC addresses and all other of the header’s fields are described in Chapter 4. The details of the network layer header fields, including the IP addresses and all other of the header’s fields are presented in Chapter 5. Finally, the details of the transport layer header, including the port numbers and all other of the header’s fields are explained in Chapter 8. In the meanwhile, some of the basic IP addressing schemes are presented in the next section, as understanding IP addressing will help us better understand the upcoming networking concepts.