At 06:42 PM 9/11/01, John Neiberger wrote:
>I think that's basically it, but I'd add two modifications.
>
>At the beginning of the process, I always find it helpful to mention
>why a station would initiate an ARP request in the first place. By
>comparing the IP address of the destination with its own IP address and
>subnet mask, it will know whether it needs to initiate an ARP request or
>send the packet to the default gateway.
Great addition and one more minor one. The station may have to ARP for the
default gateway if it doesn't already have the MAC address for the default
gateway.
>I know that you know that, I
>just like to make it clear when I'm explaining it to someone else.
>
>As for that last step about the router returning its own MAC address
>for destinations not on the local subnet, this is true only if proxy ARP
Yes, I'm glad you added that. It's important to mention that proxy ARP is
an extension to the normal behavior of ARP. Proxy ARP is sometimes called
the ARP hack. ;-) It's become accepted as normal because Cisco routers have
done it by default for so many years, but it's really kind of weird.
>is turned on AND if the originating device doesn't realize that the end
>station is not on the same local network.
We've had this discussion before!? ;-) Depending on the OS and the version
of the OS, forgetting to configure a default gateway causes a station to
ARP for everything. Also, in some versions of OSs, configuring the gateway
with your own address causes the station to ARP for everything.
Nice job, Dennis.
Priscilla
> If the originating device
>knows that the destination device is not on the local subnet, it wil not
>send a broadcast ARP. It will send a unicast packet to the default
>gateway.
>
>Regards,
>John
>
> >>> "Dennis Laganiere" 9/11/01 4:17:50 PM
> >>>
>I'm trying to describe the ARPing process. It something I've always
>taken
>for granted, but now I'm trying to actually write it down. Let me know
>your
>thoughts...
>
>When a workstation attempts to communicate with an IP address it
>follows
>this process:
>
>IP devices maintain an ARP cache that store any recently acquired
>IP-to-MAC
>address combinations. If the appropriate address is there,
>communication is
>established.
>
>If the IP address is not in the local ARP table, the source host will
>send
>an ARP request packet containing the Network-layer address, seeking to
>be
>resolve it to a MAC-layer address for the desired destination.
>
>All hosts on the network receive this request, but only the host with
>the
>specified network address will respond. If present and functioning,
>the
>host with the specified address responds with an ARP Reply packet
>containing
>its MAC-layer address. The originating device receives the ARP Reply
>packet, stores the MAC/IP address combination its ARP cache for future
>use,
>and begins exchanging packets with the host.
>
>If the host is not on the local network, the router on the local
>network
>will look up the network address in its route table and if it finds a
>match,
>it will return the MAC address of its local interface to the ARP-ing
>source
>station.
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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