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A router can connect two different networks having different underlying technology for example frame format, addressing schemes. a router can connect a LAN and a WAN , a router can connect a WAN and WAN , a router can connect a LAN and a LAN ,

Now if we have two networks that are similar to each other—means same frame format same addressing schemes , can we connect those with router instead of bridge

it seems to me that the basic purpose of router is connecting two different networks but not the networks having same underlying technology, for such network bridges are used am i right????

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Routers are layer 3 devices. Bridges are layer 2. Routers are more often used to connect networks with different underlying technologies. Both routers and bridges are used to connect networks with similar underlying technologies. –  dbasnett Jul 25 '10 at 14:01

3 Answers 3

No. Very short version: A router is used to connect any two networks and keep them apart, while a bridge is used to combine two networks into one.

And yes, you can connect two identical networks with a router. It's a standard case.

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Using the OSI model terminology, a router operates on layer 3 (the network level), a bridge (or switch) on layer 2 (the media level) and a repeater on layer 1 (the physical layer).

One problem with a bridged (or switched) network is that the "diameter" of the network must be small enough that the preamble of a broadcast can reach all devices on the network before the sending station has stopped sending the pre-amble.

So, no, you are not correct in thinking that bridges are indiscriminately used to connect network segments "just" because they happen to have the same underlying technology. This is becoming more relevant, with the use of ethernet for short- and medium-haul ISP connections.

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Bridges / Switches were introduced to overcome the diameter problem. In all shared medium networks, prior to the introduction of switches / hubs, the diameter of the network was based on the propagation of the signal in the medium. Since the protocol didn't change, the size had to. Indiscriminately doing anything can lead to problems, but connecting several switches together shouldn't be an issue. –  dbasnett Jul 25 '10 at 13:56
    
The only current Ethernet based technology that can have collisions is WIFI. –  dbasnett Jul 25 '10 at 14:10
    
@dbasnett It depends on your exact definition of "current". There's still a LOT of 100BT out there, and I've encountered hubs in them thar 100BT networks before... :-) –  Brian Knoblauch Jul 26 '10 at 20:02
    
@Brian Knoblauch - I meant current as in buy a new one today. I can't recall seeing any 100BaseT hubs. I remember when switches first appeared and they were just talking about 100BaseT. Kalpana made quite a stir at NASA HQ. A guy from Boeing Computer Services and I did a study to see if the technology worked, and was it really faster. –  dbasnett Jul 26 '10 at 21:24
    
There can also be collisions on a half-duplex link (unfortuntaly, I do see a fair few of these still in production in a variety of customer networks). –  Vatine Aug 2 '10 at 16:22

Scenario A

NetX = 192.168.1.x /24

NetZ = 192.168.2.x /24

NetX --- Bridge --- NetZ

NetX --- Router --- NetZ

Scenario B

NetX = 192.168.1.x /24

NetZ = 192.168.1.x /24

NetX --- Bridge --- NetZ

NetX --- Router --- NetZ

In Scenario A both solutions will work almost identically as long as the router is set up in the most simple manner. The big change is what layer of the protocol stack causes the packets to traverse from one collision domain to the other.

In Scenario B the bridge will work (assuming the IP's are different). The router will not.

This is a very simplistic look at what I think you were asking.

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