Imagine a scenario in which a router is connected to a local ethernet network, and to other routers on an internetwork. My question is regarding what is stored in the ARP cache on a host on the local ethernet.
As I understand it entries in the ARP cache will be of the form :-
where ip-address is the IP address of an interface on the local ethernet, and MAC-address is the physical address of that interface.
After a ping from the local ethernet to a host somewhere beyond the router, eg to a remote web server, an entry will be created in the ARP cache :-
<ip-addr of default gateway> <MAC-addr of default gateway>,
where the default gateway is the local router interface.
However on a practice CCNA test question, the entry which is made in this situation was stated as :-
<ip-addr of remote web server> <MAC-addr of default gateway>
I think this is incorrect, and only local IP addresses can appear in a host?s ARP cache. That certainly is the case on my own Windows XP PC, which is connected to the internet via a Linksys router/switch. But are there other types of host systems which store the remote IP addresses in the ARP cache ?
You are correct in what you say in that in a normal scenario the arp cache of a host will contain only entries for the local subnet. This is simply because if the host realises the address is remote it sends it to the default-gateway as mac-addresses are local to that subnet. So it should never have a remote host IP address in it's arp cache.
However in the case of proxy arp you could well end up with the IP address of a remote host being entered into the arp cache with a mac-address of the default-gateway.
Attached is a link to a doc on Proxy arp which explains how it works