Routers are the most complicated of the three devices thus far, operating at the Netork Layer of the OSI model. While bridges are limited to examining data packet MAC addresses, router go beyond this and can examine the network address–which has routing information encoded in it. Routers can use this information to make intelligent decisions about routes and paths.
In their simplest form, routers–like bridges–can be used to connect
network segments. Whereas bridges only know to forward what they don’t recognize, routers are aware of multiple paths that lead to a destination address and know which path is best.
Each network segment is assigned a specific address and is then referred to as a subnetwork or subnet. Each node on the network is then assigned an address. Every data packet sent contains the destination network address and the node address. The optimum path can then be determined by looking at the internal routing table.
One of the biggest differences between bridges and routers is the ability to identify where data is going. A router must initialize and maintain the routing is expected to be able to identify the address and only send packets for which it has a network address. If a matching address isn’t found in the routing table, the packet is discarded.
To get at the Network Layer and find the information it needs, the router must first strip off the Data Link Layer. After it finds the information, it repackages the data packet. A key advantage of routers comes into play during this operation: Since the data is unpacked and repacked, there is an opportunity to transform the data to the data frame needed for a particular arhitecture. For example, an Ethernet packet can be repackaged and sent over a token ring network by the router’s unpacking the Ethernet data frame and repacking with a token ring frame.
Almost every router made today can support multiple protocols. Here are the most common (in alphabetical order): CLNP, DECnet, IPX/SPX, PPP, PPTP, SLIP, SNA, SNAP, TCP/IP, X.25, XNS Protocols
While this list is impressive, there are some noticcable absences, because some protocols can’t be routed: DLC, LAT, NetBEUI
