ARP Cache On Host

Answered Question
Aug 25th, 2007

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 :-

<ip-address> <MAC-address>

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 ?

I have this problem too.
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Correct Answer by Jon Marshall about 9 years 3 months ago

Hi

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

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml

HTH

Jon

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Correct Answer
Jon Marshall Sat, 08/25/2007 - 04:52

Hi

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

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080094adb.shtml

HTH

Jon

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