Question about MAC address rewrites and packet flow.

Do MAC address rewrites occur EVERY time a packet crosses a switch or are there any exceptions to the rule? Little confused about this issue so I need some clarification. I know for sure that MAC address type stuff is only used at the LAN level. Past the lan level it turns to straight DNS/IP address.

If anybody can shed some light or comment on this I would greatly appreciate it. Just trying to make sure I understand what happens to traffic as it travels from one pc to another pc accross a LAN, and also accross a WAN.

Thanks.

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Comments

EdTheLad

A switch doesnt rewrite the mac address,it just forwards the frame to the mapped port.If you need to communicate between two different subnets a router is needed.A packet from the pc will have a destination mac address of the default gateway i.e. the router, when the router receives the packet it will strip off the layer 2 info,check its routing table and re-encapsulate the data with a new layer 2 header i.e. the mac is rewritten

Webmaster

Indeed, the switch doesn't change the mac address as it remains in the same Ethernet LAN / layer 2 broadcast domain.

mtbcyclist wrote:

Just trying to make sure I understand what happens to traffic as it travels from one pc to another pc accross a LAN, and also accross a WAN.

www.faqs.org/rfcs/rfc1180.html
Yes, an RFC, don't worry it's a special one. Probably the clearest description available for traffic flow process in TCP/IP networks.

mtbcyclist

Thanks guys. I am trying to brush up on my LAN knowledge by reviewing a bunch of stuff. I RARELY work at the lan level as I am a WAN engineer. Due to that fact I like to do a review from time to time and could not find anything clarify this in my old study notes. Thanks for the RFC info, as I will most definitely read it.