If i understand correctly:
1. You have is 10 gateways for 10 machines
2. And you want to interlink the 10 machines like a LAN!
I have a serious doubt whether things will work incase you buy a second LAN card for all machines. It probably wont. In case you buy a second LAN card for all machines you can still only give one IP as a default gateway, which in most cases will be 192.168.1.1 (MTNL router) if I am right. the second LAN card will be in another network and even if you set the NIC priority right you are likely to face issues either while browsing or while playing games!!
All this assuming you are not going to change the default gateway each time.
Now coming to the switch which you are talking about:
are you going to buy a hub or a switch?
the ones which you get for 1000INR in the market are hubs and not switches and will not have an IP of their own.
Consider this case. You dont need second LAN card for all PCs & you will not need a server.
1. All MTNL routers have four LAN ports (correct me if I am wrong)
2. MTNL routers give you an option of changing their own IP (Default is 192.168.1.1)
3. For the 10 routers: Assign IPs from 192.168.1.1 to 192.168.1.10 (subnet 255.255.255.0)
4. For each PC Assign IPs 192.168.1.101-192.168.1.110
5. For each PC give the default gateway as their respective routers
6. Buy two 8-port hubs or one 16 port hub (i dont know the cost of a 16 port one) and do the connection part from each of the router to the hub and interconnect the hubs using straight LAN cable.
7. Of course you need to connect from PC to your own router.
8. If MTNL routers allow filter in such a way that they accept traffic from only one MAC then your problem of internet being hacked is solved!! (Change default password)
If they dont, block MACs of the other 9PCs in the network (still there is a risk)
Cant think of anything about this as of now
9. Make sure all NICs are configured for 100MBps full duplex and buy good cables.
10. Resolve the machine names of all the PCs using hosts file on each machine.
This should work on paper. Do a small test. Connect two PCs with a cross cable and use a software to check how much of data transfer takes place after half an hour of two player gaming.
Considering you have only 10PCs this model should work (assuming i have understood your requirement correctly)
Correct me if any of my assumptions are wrong.
*img62.imageshack.us/img62/2431/lanrn.th.jpg
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hope this helps