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Gateways vs. routers/bridges?

What are gateways and how do they differ from routers and bridges?
Bridges broadcast data across an entire network, they do not route and therefore are protocol independent. The benefit is sending data packets across networks with different protocols, the downside is the amount of traffic sent across a wide area network (WAN) because of this broadcast. Years ago, when most companies used proprietary protocols, this helped get data across the WAN from one point to another. For example, one Netware network using IPX could bridge data across to another Netware segment, but all non-Netware segments would also receive these packets even though they couldn't interpret them. Today, bridges are rarely used.

Routers on the other hand, actually route data to a specific location based on a address for the network segment, the benefit is the ability for a router to search it routing tables and find the shortest path to the destination. The downside to routers is that they are protocol dependant and therefore can only route data between network segments using the same protocol. Today this is a moot because everyone uses TCP/IP and has an open architecture. This is why for example, data can be sent between a Windows NT network and a Netware network. The way a router works, is that when it receives a packet and sees a MAC address (hardware address) that is not on the local segment it strips away the MAC address, looks at the IP address (software address), searches it's routing table and sends the packet based on the IP address to the router that's connected to the segment that contains that address.

Gateways actually do protocol conversion, since everyone uses TCP/IP these days, gateways are also not used that often. For example, years ago, if you wanted to connect a PC to an IBM mainframe, you would typically got through an SNA gateway that would covert the protocol your PC was using (i.e. IPX) to SNA so that the packet could be understood by the mainframe, this same gateway would convert SNA packets coming back form the mainframe to IPX so the PC could understand it. Again, since today everyone uses TCP/IP this is rarely a used method today.

This was last published in June 2002

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