This tutorial is about the elusive bridging technology so commonly employed in ADSL routers and stuff but not so generally used in TCP/IP networking.
I wanted to talk about VLANs but I have no experience with them. Perhaps later. For now let us focus on bridging. What is a bridge? Since I grew up in the Lakshadweep islands the word bridge only meant one thing. No, not the connection across a river as there are no rivers in Lakshadweep. The topmost part of the ship/liner where the captain sits and where they drive the ship. I have also driven ships and boats. ;) Anyway now the meaning of a bridge is not something we come across often in networking; it is supposed to be a layer II device that does switching; no routing. Somewhat like a hub/network switch. Now a computer with sophisticated routing capabilities can jolly well act as a dumb switch. And switches talk a protocol called STP , spanning tree protocol which is nothing but a way to regulate the traffic. In switched mode packet transmission it is very important. Alright, now let us focus on software and our work. Say we have a situation where we have to extend a network segment between wired and wireless world or even physically across. Obviously creating a different subnet/mask for that is problematic. To keep the network less complicated we can resolve to bridging in which a computer acts as a simple physical interconnection of networks such that the topology does not change. A very good example is my house office. In case I wish to have a LAN downstairs with WiFi upstairs I will use bridging. Now bridging is also employed in ADSL MODEM testing. I have a buddy in BSNL who keeps bothering me all the time with some MODEM that is not configured correctly; he with either have the DHCP address wrong or some stupid mistake in the MODEM will trouble him. And I have always found that people with little ability are seldom benefited by help. Anyway he normally keeps the MODEM in bridge mode and runs a dialer from the Windows desktop to connect to the Internet. What is a MODEM? It is a firewall, router, DHCP server, NAT all in one. What is the advantage of bridging? Except that you can run the dialer from a PC instead of the MODEM itself which talks PPPoE there is no big advantage as such. But it is always good to have an option. A MODEM has an ethernet port, a Wifi port and an RJ11 phone jack which connects to the CO. The public IP is available in the phone network. The local ethernet network has a local IP. The MODEM can only route traffic between the two networks. But you can ask the MODEM to only do the physical connectivity to the exchange server and let the Windows desktop do the PPPoE dialing with username and password. In this case,the ethernet or Wifi port does the work of the phone line to talk to the exchange. It is a very interesting concept if you think of it... -Girish -- Gayatri Hitech http://gayatri-hitech.com _______________________________________________ ILUGC Mailing List: http://www.ae.iitm.ac.in/mailman/listinfo/ilugc
