Routers

Packets are only passed to the network segment they are destined for. They work similar to bridges and switches in that they filter out unnecessary network traffic and remove it from network segments. Routers generally work at the protocol level. Routers were devised in order to separate networks logically. For instance, a TCP/IP router can segment the network based on IP subnets. Filtering at this level ( on IP addresses) will take longer than that of a bridge or switch which only looks at the MAC layer.

Most routers can also perform bridging functions. A major feature of routers, because they can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving the network.

Typically, an organization which connects to the Internet will install a router as the main gateway link between their network and the outside world. By configuring the router with access lists (which define what protocols and what hosts have access ) this enforces security by restricted (or allowing) access to either internal or external hosts.

For example, an internal WWW server can be allowed IP access from external networks, but other company servers which contain sensitive data can be protected, so that external hosts outside the company are prevented access.