In the example shown in Figure 5-1, and using Internet terminology, an application header is added to a message on the application layer (for example, an email). This message is passed to the TCP/UDP layer, where it gets a TCP or UDP header (TCP in the case of an email). On the IP layer, it receives the necessary information to find its destination, and, most important, the IP Address of the destination. On the link layer, a header is prepended that contains the physical addresses for a Local Area Network, mainly, the Ethernet addresses of the sender and the receiver, and a trailer that contains error-checking information. The physical layer represents the actual signal on the media.

See Also: Electronic Mail or Email; Ethernet; Internet; Internet Protocol (IP); IP Address; Local Area Network (LAN); Message; TCP/IP or Transmission Control Protocol/Internet Protocol; User Datagram Protocol (UDP).