ARP Frame Structure

ARP frames use the EtherType of 0x0806. ARP is not a client protocol of IP, and ARP frames do not contain an IP header. Thus, ARP is useful only for resolving MAC addresses for IP addresses that are on the same physical subnet, the boundaries of which are defined by IP routers. IP routers never forward an ARP Request or ARP Reply frame.

As RFC 826 describes, an ARP frame’s structure suggests that ARP could be used for MAC address resolution for protocols other than IP. However, in practice, IP is the only protocol that uses the ARP frame format. Figure 3-1 shows the structure of the ARP frame for the IP protocol and for LAN technologies that use a 6-byte MAC address.

Figure 3-1. The structure of an ARP frame

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