From: Paul Jakma (paulj at domain alphyra.ie)
Date: Mon 13 Aug 2001 - 14:21:53 IST
On Mon, 13 Aug 2001, Conor Daly wrote:
> Even when handling only one IP link to receive X frames?
on ethernet, yep. (and presumably same for IP on other multi-access
to talk to another node you need it's MAC.
to find out what MAC you need to talk to node with IP addr <foo>, a
protocol called ARP is used it goes like:
"hey, whose got <IP:foo>? tell <IP:bar>"
"hi, <IP:foo> is <MAC:mac>"
and <IP:bar> remembers the <IP:foo> -> <MAC> mapping and uses that.
however, nodes only remember that association for a small lenght of
time, iirc 30 seconds. (if you remember for a long period of time,
then thngs like nodes changing IP addresses will not work very well -
eg DHCP environment). Hence, nodes need to regularly use ARP, as they
forget previous ARP replies within 30 seconds (ok, maybe not 30 - but
something in that order). eg on linux cat /proc/net/arp to look at the
current state of the ARP cache.
ARP is a broadcast protocol, both the request and reply are sent to
the hardware broadcast address.
(quietly awaiting the inevitable followup postings to correct my
invariably wrong mumblings. : )
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