On Wed, 2004-07-21 at 14:37, Benjamin J. Weiss wrote: > MAC addresses are layer 2, not layer 3. This means that they're used > on the same subnet, but don't cross a router. IIRC, MAC addresses don't > cross switches either, only hubs or bridges. Actually, MAC addresses are bounded generally by Layer 3 or higher devices. Hubs are multi-port repeaters that some would say operate at the physical layer (Layer 1). Bridges and Layer 2 switches (aka. multi-port bridges) operate at Layer 2. What the "transparent" variety do for us is simply be selective in forwarding based on MAC address (though will "flood" broadcasts and multicasts), so the MAC address must be unique within that "broadcast domain". Now routers, and Layer 3 switches will strip the MAC header and create a new header as the packet is transmitted between "broadcast domains" or "subnets", perhaps represented by different "VLANs". (Yes I know the high end routers and layer 3 switches might do an inline rewrite to speed things up, but....) In reality, the two "sides" of the Layer 3 device can actually be different Layer 2 technologies.... HTH, --Rob
