A network switch is a networking device that connects network segments. Technically, network switches operate at layer two (Data Link Layer) of the OSI model. They were developed from the electronic hub, where the hub provided a central nodal device for a star-configured network. In a shared hub, all star network connections receive a broadcast frame. A switch is similar to a hub in that it provides a single broadcast domain, but differs in that each port on a switch is its own collision domain. Generally, a switch contains more "intelligence" than a hub. Network switches are capable of inspecting the data packets as they are received, determining the source and destination device of that packet, and forwarding that packet appropriately. By delivering messages only to the connected device that it was intended for, network switches conserve network bandwidth and offer better performance than hubs.

Switches make it difficult to monitor traffic because each port is isolated until it transmits data and even then only the sending and receiving ports are connected. Two popular methods that are specifically designed to allow a network manager to monitor traffic in a switched environment are:

• port mirroring C the switch sends a copy of network packets to a monitoring network connection;
• SMON C "Switch Monitoring" is described by RFC 2613 and is a protocol for controlling facilities such as port mirroring.

Other methods have been devised to allow snooping on another computer on the network without the cooperation of the switch, such as ARP spoofing an MAC Flooding.

The most popular network switch in the LAN environment is the Ethernet switch. In the WAN network, ATM switches, Frame Relay and 10 Gigabit Ethernet switches are often used. For high end WAN switches, a (soft) router is often included in the switch to provide network layer (layer 3) packet processing.

Network Switch

Related Terms: Hub, Router

Reference Links: http://computer.howstuffworks.com/lan-switch.htm: How a LAN Switch works