[net.lan] What is ARP?

dfh@SCIRTP.UUCP (David F. Hinnant) (08/14/85)

  I'm looking for information on the Address Resolution Protocol:

	- What it is
	- Where it fits in to the scheme of things
	- When do I need it
	- etc.

Any comments appreciated.  Pointers to printed or electronicly readable
documentation would be ideal.

				As they say,  Thanks In Advance.
-- 
				David Hinnant
				SCI Systems, Inc.
				{decvax, akgua}!mcnc!rti-sel!scirtp!dfh

root@bu-cs.UUCP (Barry Shein) (08/18/85)

ARP (Address Resolution Protocol) is simply a method of mapping hardware
addresses to 'logical' addresses, particularly in an ethernet.

Every ethernet board has a 48-bit address assigned to it when it is
manufactured, if all has been going well every single such board is
unique and is not settable (ok, some are, but forget that fact!)

A network using addressing like the DARPA/INTERNET uses 32-bit
addresses, usually thought of as 4 1-byte fields. At some low level
someone must map the desired internet address to a 48-bit hardware
address.

You could build a table:

hosta	126.5.3.12	00.AA.00.12.55.C0

etc for each host. Systems that do not use ARP do this (note that not
all hosts reachable on your network should be constrained to be ethernet
hosts so the 'logical' address and a way to map it is useful rather than
trying to accomodate all possible hardware interfaces with a one level
scheme.)

With ARP what we do is send a broadcast packet on the net requesting
that the host who is 'logically' 126.5.3.12 (or, perhaps, someone who
will forward to that host) please respond to me with their hardware
address so I can put it in a temporary table for future (but not
permanent) reference. I will do this every time I do not know how to
talk to this host.

One advantage is that if the remote host's hardware interface breaks and
we need to replace it no human needs to go replace tables, just let the
machines re-ARP, the logical addresses will stay the same.

At BU we use an ARP protocol on our broadband hardware as well as our
ethernet hardware. It only makes sense if your network hardware supports
a broadcast (wildcard) address.

	-Barry Shein, Boston University