Say we have a router that connects to other routers and they connect to lan networks, does that router have a network card for each router it connects with but a single forwarding table for all? or even each network card has its own forwarding table?

Or a more simple case where a router that connects between two LANs like in the picture below.

Since we see that each side has a different mac address, I'm led to believe that it has at least two network cards, but wouldn't that mean if we have for example a router that connects hundreds of other routers or networks, does it also have hundreds of network cards?

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2 Answers 2


A router (or routing instance) will have a single layer-3 forwarding table for all interfaces, not one per NIC. Except for an aggregation router as used in an ISP (which will not necessarily be Ethernet, so your ideas about NICs probably don't apply), most routers (as opposed to L3 switches) will have only a few or a few tens of IP subnets attached.

Where you may find very many subnets is with a L3 switch such as in a large data centre, and these may have hundreds of ports (possibly distributed across multiple racks) but will also have dot1q VLAN tags on many ports as mentioned by Viktor. However in this case the L3 interface is not associated with any physical port, and the MAC address used for each L3 interface will be defined by software.


Single card can have multiple interfaces. In addition to that on a single interface you can use do1q tagging and transport multiple services over 1 physical link.

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