On Thu, 2008-08-21 at 20:31 +0800, Justin Richards wrote:
> Hi Folks,
> 
> I am experincing conflicting results on what appears to be identicle
> setups as follows:-
> 
> 1. Any destination ip address handled by the gateway device does NOT
> result in an arp entry but all local ip address are added.
> 2. Any destination ip address handled by the gateway device does
> result in an arp entry but all local ip address are added.

Please correct me if I'm wrong, but I think you're confused as to the
purpose of "arp".

ARP is only used for "local" addresses, that means IPs that match your
subnet mask.

So, say your IP is 192.168.1.1, your default gateway is 192.168.1.254,
and your subnet mask is 255.255.255.0

If you try to send something to 192.168.1.45 the following occurs:

1) The subnet mask check will result in 192.168.1.45 being identified as
being on your local subnet.
2) An ARP request will be sent out asking for the MAC of the owner of
192.168.1.45
3) An ARP response will be received, the MAC will be added to the local
table, and the packet will be sent to that MAC.

Now, if you try to send something to 192.168.2.45:

1) the subnet mask check will result in 192.168.2.45 NOT being on the
local subnet
2) as a result, the packet will be sent to the default gateway
192.168.1.254. If the default gateway's MAC isn't "fresh", and ARP
request will be sent for the GATEWAY's IP, NOT 192.168.2.45. 
3) the packet destined for 192.168.2.45 will instead be sent to the MAC
of the gateway, which we assume will forward it on to wherever it needs
to get to.

Basically, anything on your local domain is sent directly to that
machine, anything else is sent to the gateway to your domain, which will
then forward it on as necessary in order to reach the final desination.

Note: I am ignoring some more "complicated" network configs that could
muddy up what I've described. Also, VPNs are a special case where all
this sort of stuff becomes a little more complicated (although not much
more so).

TTYL

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