The ARP Details

The ARP protocol has several fields that parameterize its use in any specific context [2]. These fields are:

         hrd     16 - bits       The Hardware Type Code
         pro     16 - bits       The Protocol Type Code
         hln      8 - bits       Octets in each hardware address
         pln      8 - bits       Octets in each protocol address
         op      16 - bits       Operation Code

The hardware type code assigned for the IEEE 802 networks (of all kinds) is 6 (see [7] page 16).

The protocol type code for IP is 2048 (see [7] page 14).

The hardware address length is 2 for 16-bit IEEE 802 addresses, or 6 for 48-bit IEEE 802 addresses.

The protocol address length (for IP) is 4.

The operation code is 1 for request and 2 for reply.

Broadcast Address

The broadcast Internet address (the address on that network with a host part of all binary ones) should be mapped to the broadcast IEEE 802 address (of all binary ones) (see [8] page 14).

Trailer Formats

Some versions of Unix 4.x bsd use a different encapsulation method in order to get better network performance with the VAX virtual memory architecture. Consenting systems on the same IEEE 802 network may use this format between themselves. Details of the trailer encapsulation method may be found in [9]. However, all hosts must be able to communicate using the standard (non-trailer) method.

Byte Order

As described in Appendix B of the Internet Protocol specification [1], the IP datagram is transmitted over IEEE 802 networks as a series of 8-bit bytes. This byte transmission order has been called "big-endian" [11].

Maximum Transmission Unit

The Maximum Transmission Unit (MTU) differs on the different types of IEEE 802 networks. In the following there are comments on the MTU for each type of IEEE 802 network. However, on any particular network all hosts must use the same MTU. In the following, the terms "maximum packet size" and "maximum transmission unit" are equivalent.