MAC addresses question

[robvanderlinden]

I'm confused about the whole MAC addressing thing. On a WAN, is it that the MAC address is included in segments and sent passed

the router and then being forwarded to the host or is it so that the MAC address does not go any further than the router and that the router

looks up the right ip address for the corresponding mac address (and if it doesn't know it, it will use ARP)?

[pvezina]

Hello Rob,

Every time a packet passes through a router, the source and destination MAC addresses will change, which makes MAC addresses locally significant only. If we assume that there is no NAT involved in the conversation then the source and destination IP addresses will always stay the same.

So when a router receives a packet, it looks at the destination IP address, and makes a decision on where to send that packet based on the routing table. Let's say for example that the final destination host is not directly connected to this router. Then this router (Router A) will send the packet to another router (Router B, the next-hop in the routing table). Router A will use Router B's MAC address as the destination MAC, and the MAC address of his output interface as the source MAC. If Router A does not know the MAC address of Router B, then it will send out an ARP Request to get that MAC address. Router A will not modify the source or destination IP addresses in the packet, only the MAC addresses.

This is based on the WAN being Ethernet. If you have other technologies (PPP, HDLC, Frame-Relay or ATM) then the WAN link will not use a MAC address at all. For example with Frame-Relay it uses a DLCI to identify the circuit being used. The IP adressing at layer 3 will work over the Layer-2 technology being used.

Hope this helps you understand.

Patrick.