Ethernet is the typical transport layer protocol in IP networks. It is designed to transmit packets from a sender to a receiver, both identified on behalf of an address information element.
According to this limited functionality, Ethernet has a very small header. The header field (Figure 1) contains only the information elements:
- Destination address.
- Source address.
- Ethernet type.
Ethernet type is similar to a SAPI (Service Access Point Identifier). It contains information about which higher layer protocol information is transported by Ethernet frames.
The Ethernet addresses, often called MAC addresses (but with nothing in common with RLC/MAC!), consist of 6 bytes. These MAC addresses are fixed hardware addresses and, due to a defined numbering scheme, each address is unique worldwide.
If IP data is to be transmitted using Ethernet the hardware MAC address of the receiver of IP packets is unknown when the connection starts. Only the target IP address is known. However, since Ethernet is the lowest layer of the connection there must be a source and a destination MAC address included in each header. In other words, for each sender IP address there is an appropriate sender hardware address, and for each destination IP address there must be an appropriate target hardware address.
The target hardware address that is related to the target IP address is requested by the Address Resolution Protocol (ARP). Its sister protocol, the Reverse Address Resolution Protocol (RARP), can be used to find the target IP address (or, in terms of ARP/RARP, the target protocol address) to a known MAC address.
The address resolution procedure consists of two steps:
- ARP request (req) message with Target Hardware Address = "0" is sent to all(!) IP clients in the network.
- The client that has the target protocol address as set in the ARP req message sends ARP Replay (rpl). The sender hardware address in ARP rpl is the Ethernet MAC address related to the destination IP address that the sender of the ARP req is looking for. An example of the Ethernet address resolution procedure is shown in Figure 2.