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Given a host A und B within the same broadcast domain whose ARP caches are both empty, under what circumstances would B add an entry for A to its ARP cache when replying to an incoming ARP request received from A?

As far as I understand all the information required is contained in the ARP header. But I think I can remember a situation a few years ago where B has not used this information and initiated an ARP request on its own to get back to Host A (e.g. sending ICMP echo reply).

Thanks in advance.

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Whenever a node needs to contact some other node, it checks its ARP cache. If the MAC address is found in the host cache, it uses it. Otherwise, it sends a broadcast to its subnet asking form the MAC address of host X. The ARP reply is used to get the MAC address and it saves it in its cache for future use.

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It doesn't actually broadcast to it's subnet, it broadcasts to the limited broadcast address of, which is intended for every node connected to the same physical segment (layer 2), as opposed to the subnet directed broadcast address (layer 3), which is intended for all nodes in the same layer 3 subnet (as there may be more than one layer 3 network in the same physical segment. – joeqwerty Jan 12 '12 at 13:50
But will the node which receives the ARP request also created an ARP cache entry from the ARP request package? – aka Jan 12 '12 at 18:55
Yes it will.... – joeqwerty Jan 17 '12 at 1:39

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