4.4 ARP Packet Format

Figure 4.3 shows the format of an ARP request and an ARP reply packet, when used on an Ethernet to resolve an IP address. (ARP is general enough to be used on other networks and can resolve addresses other than IP addresses. The first four fields following the frame type field specify the types and sizes of the final four fields.)

Figure 4.3. Format of ARP request or reply packet when used on an Ethernet.

The first two fields in the Ethernet header are the source and destination Ethernet addresses. The special Ethernet destination address of all one bits means the broadcast address. All Ethernet interfaces on the cable receive these frames .

The 2-byte Ethernet frame type specifies the type of data that follows . For an ARP request or an ARP reply, this field is 0 —0806.

The adjectives hardware and protocol are used to describe the fields in the ARP packets. For example, an ARP request asks for the hardware address (an Ethernet address in this case) corresponding to a protocol address (an IP address in this case).

The hard type field specifies the type of hardware address. Its value is 1 for an Ethernet. Prot type specifies the type of protocol address being mapped. Its value is 0 —0800 for IP addresses. This is purposely the same value as the type field of an Ethernet frame containing an IP datagram. (See Figure 2.1.)

The next two 1-byte fields, hard size and prot size, specify the sizes in bytes of the hardware addresses and the protocol addresses. For an ARP request or reply for an IP address on an Ethernet they are 6 and 4, respectively.

The op field specifies whether the operation is an ARP request (a value of 1), ARP reply (2), RARP request (3), or RARP reply (4). (We talk about RARP in Chapter 5.) This field is required since the frame type field is the same for an ARP request and an ARP reply.

The next four fields that follow are the sender's hardware address (an Ethernet address in this example), the sender's protocol address (an IP address), the target hardware address, and the target protocol address. Notice there is some duplication of information: the sender's hardware address is available both in the Ethernet header and in the ARP request.

For an ARP request all the fields are filled in except the target hardware address. When a system receives an ARP request directed to it, it fills in its hardware address, swaps the two sender addresses with the two target addresses, sets the op field to 2, and sends the reply.