Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 64
Rules for ARP

1.  ARP is not a part of the IP protocol and therefore does not contain IP headers. ARP works directly on top of the datalink layer.
2.  ARP requests and responses are transmitted with a destination physical broadcast address (all Fs) and therefore never leave their logical subnet. Plus, with Rule 1, these packets cannot be routed
3.  Since ARP is not part of the IP protocol, a new EtherType (the field in the Ethernet packet that identifies the protocol used by the packet) is assigned to identify this type of packet. 0806 is an ARP request and 0806 is an ARP reply. Some ARP implementations can be assigned the 0800 EtherType, for IP will be able identify the packet as an ARP request or ARP reply packet. Not all implementers of IP use these types. Some still use the EtherTypes of 0800 for ARP.

Rules for ARP

•  ARP does not run on top of IP and therefore has no IP headers.
•  ARP requests are transmitted in broadcast so that all stations receive the packet.
•  New EtherType defined 0x0806 for both the ARP request and reply.
•  ARP replies are sent directly to the requesting station (unicast, not broadcast).
•  ARP tables should age out their entries.
•  An attachment should answer an ARP sent to itself.
4.  Some implementations have an ARP aging capability. This allows ARP to delete entries that have not been used for a period of time, reducing the ARP lookup time and saving memory.
5.  If a machine submits an ARP request for itself, it must reply to the request.


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