May u give brief knowledge how router work?

How does it forwards or map ip in different subnet simplely.

What is the definition of it and how is the basic function?
turbot_yuAsked:
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Steve KnightIT ConsultancyCommented:
Each interface has an IP address.  In that basic state the router will take any packets that arrive on one interface with a destination address on the same subnet as another interface and send them out to it.  Client devices generally reach a router through their default gateway setting or from being being redirected there by another network device (another router or L3 switch).

One interface may have a default gateway, any packets that come into the router that it doesn't have a specific route for go out that interface.

The router builds up routing tables either defined manually (static routes) or automatically by talking to other router devices so that it knows out of interfaceA it can get to 10.1.0.x and out of InterfaceB it can to get 192.168.x.x for instance.

http://en.wikipedia.org/wiki/Router
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turbot_yuAuthor Commented:
Thanks a lot, quite clear,
May u give a bit more on
'Client devices generally reach a router through their default gateway setting or from being being redirected there by another network device '

And how and where to set the gateway.
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Steve KnightIT ConsultancyCommented:
'Client devices generally reach a router through their default gateway setting or from being being redirected there by another network device '

OK, this is basic TCP/IP stuff and there are many tutotials out there, but will give a brief run down.

Each PC has an IP address, and subnet mask as a minimum.
The IP address and subnet mask are combined to identify if another IP address is on the same subnet and therefore contacted directly or on another subnet in which case to be sent to the default gateway.

e.g.


your ip is 10.0.1.2 and subnet mask is 255.255.255.0  defult gatrway is 10.0.1.1
router ip is 10.0.1.1
other ip #1 is 10.0.1.100
other ip #2 is 10.0.2.100
other ip #3 is 220.1.220.1

to get from 10.0.1.2 to 10.0.1.100 it is on the same subnet so it sends direct
to get from 10.0.1.2 to 10.0.2.100 it is on a different subnet so it sends to 10.0.1.1 which may or not know how to get to 10.0.2.100.  Same with 220.1.220.1


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turbot_yuAuthor Commented:
At least router will have 2 ports, does each one has a ip of gateway?
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Steve KnightIT ConsultancyCommented:
Not normally.  The gateway you mention is the default gateway and therefore where any unknown traffic is sent.  If multiple interfaces have a default gateway where do you send a packet?

Normally the router will either learn from other routers or have speciifc routes programmed into it so that it knows what is out each interface.

The one tiime this might not be the case is if the router has tow indepenedent routes through different internet connections in which case both would have a default gateway and some for of load balancing would be needed to decide which one was used.
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turbot_yuAuthor Commented:
So multiple interfaces have a multi gateway, each interface has a gateway for itself, right?

How does the router make the traffice direction in router as double direction?
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Steve KnightIT ConsultancyCommented:
If a packet comes in anywhere then the router checks tis routing tables to determine wher to point it.  Type ROUTE PRINT from a CMD.EXE console on a Windows box to get an idea of what it uses to end packets different ways.
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Steve KnightIT ConsultancyCommented:
Thanks for the point, glad it helped
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turbot_yuAuthor Commented:
Thanks a lot, Any simple and good book I can go further?
:))
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Steve KnightIT ConsultancyCommented:
Don't know sorry, learnt all this 10-15 years ago :-)   Bit of experience, set up some test network then get hold of a CCNA Cisco course perhaps?
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