Subinterface

In telecommunications and computer networking, a subinterface is a division of one physical interface into multiple logical interfaces. Routers commonly employ subinterfaces for a variety of purposes, most common of these are for routing traffic between VLANs, and in Non Broadcast Multiple Access networks such as frame-relay or ATM.

Routing between VLANs with subinterfaces

Because VLANs are functionally equal to multiple separate switches each with its own subnet, a router is required to route traffic between them. Networks that have multiple switches connected to a router of course need one physical connection from each switch to a separate port on the router. The same would normally be true of VLANs within a single switch, you would need a separate physical connection from a port in each VLAN on the switch, to multiple ports on the router. This becomes a problem for a number of reasons, but primarily because most routers don't have more than a few Ethernet ports, and even with enough ports this would be an unnecessary waste of resources on the router. The solution to this multiple connection problem is to use what is called a trunk line.

Non Broadcast Multiple Access

In an NBMA network, broadcast or multicast packets are not passed between circuits. This presents problems for traffic using these packets such as routing updates. To solve this problem, subinterfaces are sometimes used to allow for each logical circuit in a physical connection to be considered as its own interface, thus allowing for routing updates and other non-unicast traffic to be passed between them, which is otherwise not possible.