Knowledge of destination MAC address

You are looking for something called Address Resolution Protocol (ARP), for IP Version 4, or Neighbor Discovery (ND), for IPv6. Respectively,

and

These are used when the destination is on the same IP subnet as the sending host. If the sending host sees that the destination IP address is not on its own IP subnet, then the destination MAC address is the address of the default router (typically).

ARP is not IP-specific, by the way. It can accept any Layer 3 addresses, in principle.

Bert

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In principle, but do you know any other than IP that use it?

Apple's ethertalk uses AARP. I don't know where an online reference is. See "Inside AppleTalk, second edition".

DECNet assigns the MAC address based on the DECNet area and node.

-- glen

Thanks Bert. It was quite useful. I was aware of the ARP but not that it is not specific to IP.

If you can help with another question.. it would be great!!

Now this desitnation address.. is it a part of the data sent by the layer 3 or is there a specific format for sending the same?A similar question would arise for the length/type field of the MAC frame structure.

Thanks in advance.

Aswin

You're welcome.

Not sure if I understand the question. The returned MAC address goes into an ARP table, and is stored in the sending host for a few minutes. You can see this table in Windows machines by typing "arp -a" at the command line prompt. In fact, if you ping some host you know exists in your IP subnet, you can then measure how long the entry remains in the table. Just keep doing "arp -a" until you see the entry disappear.

When the layer 3 protocol sends a packet down to the Ethernet driver (assuming Ethernet), that's when the ARP table is checked, and that's when the MAC destination address is entered into the Ethernet frame. Or, if the destination's MAC address is not in the table, then the ARP query is sent first, and the IP packet is held up until the ARP reply comes back.

Bert

No, but that's why I said "in principle." For example, even IPv6 *could* use ARP. But IPv6 has its own unique mechanism.

Another point is that ARP gets a lot of bad press for using broadcasts for its queries. But once virtually everything was migrated to IPv4, I don't think it made a whole lot of difference anymore. There were not many hosts out there that got interrupted unnecessarily. ND, with its multicast queries, will eventually interrupt all hosts too, if IPv6 becomes the norm.

DECnet had some sort of address resolution protocol. I forget the name. You had to discover all the nodes on your network, but it was a one time deal. Remember? It took a few minutes to populate the table.

Bert

Maybe I'm confusing that with LAT?

Bert