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

Nonbroadcast Multiple Access Network

A non-broadcast multiple access network is a computer network to which multiple hosts are attached, but data is transmitted only directly from one computer to another single host over a virtual circuit or across a switched fabric. NBMA networks do not support multicast or broadcast traffic...

 networks such as frame-relay

Frame relay

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...

 or ATM

Asynchronous Transfer Mode

Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...

.

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.

A trunk line aggregates traffic from multiple independent VLANs into a single physical connection between switches, or between a switch and a router. Within this trunk there is a logical division of the connection, by encapsulating each frame with VLAN information, or by using a special frame header marking each frame as belonging to a specific VLAN. Normally this trunk will carry traffic for each VLAN present on the switch.

However routers only have one Ethernet MAC address per port, and normally each port is part of a single IP subnet. This means a solution must be found to allow the router to route traffic on a single port as if it were part of multiple separate IP subnets, and this is where subinterfaces come in.

As an example of what a typical subinterface setup might look like on a Cisco router, a single Ethernet port such as FastEthernet0/0 will be subdivided into Fa0/0.1 , Fa0/0.2 , Fa0/0.3 and so on, each one performing as if it were a separate interface. Each of these subinterfaces corresponds to a single VLAN on the switch.

Non Broadcast Multiple Access

In an NBMA network, broadcast

Broadcasting (networks)

right|250pxIn telecommunication and information theory, broadcasting refers to a method of transferring a message to all recipients simultaneously...

 or multicast

Multicast

In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...

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.