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3. Routing

3.1 Required hardware

Off course this all depends on your traffic. Asuming that you are setting up a router for a small company, let's say about 100 PC's, a 486SX will do well. I'm sure you'll find this PC somewhere in your company just sitting in a corner because it can't cope with the popular OS's like Windows '95 or OS/2. Put 16MB of RAM in this PC, a harddisk of 170MB, and you are ready to install your router.

3.2 What is routing?

Let us take a look at my own configuartion:

  \                                                         --------
   \                                   | host |
    \                     ---------                       |-|      |
     |    14400 bps       | Linux |       10Mbit          | --------
 NET =====================| masq  |-----------------------|
     |    PPP/slip        | router|   TCP/IP backbone     | --------
    /                     ---------                       |-| host |
   /                                                        |      |
  /                                                         --------

The intern network is based on a private Class C network. The netmask is You can see that if host wants to connect to that there is no problem. But if host wants to connect with, there's a problem, because is not located in his own network. For this problem there's a solution, set-up a gateway ( and put a default route in So if wants to make a connection to a computer who he can't find find in his own network (, he has to contact those computers trough the gateway. A default route can be added with the next command to a Linux system.

route add default gw eth0

The 'gw' argument tells the route command that the next argument is the IP address, or name, of a gateway or router machine which all datagrams matching this entry should be directed to for further routing.

Most popular Linux systems use /etc/sysconfig/network to get the default route at boot-time. If you use such a distribution, just put the following lines in it.


Now to answer the question above, 'What is routing?', I would say that routing is the process of delivering packages from PC1 on network A to PC2 on network B.

3.3 Why does the Internet uses routing

When setting up the technology for the Internet (first known as ArpaNet), the US-army wanted a network that could function in very difficult situation (like war ;-). They wanted a network which wouldn't go down when a server was hit. With other words, they wouldn't want a network with a central command. They wanted to create a network in which computers could connect and won't fall out when a server crashed.

Routing was invented for this purpose. Routing enables you to connect different networks. Let's assume there's a network called A and there's a network called B. With the TCP/IP technology, network A and network B can function without each other. Routers can connect these individual networks, but it is no problem when the routers go down, the 2 network will be seperated from each other, but thats all.

Mayby I can explain this with an example for those under you who use the IRC-networks. You can login on (for ex.) This server is connected trough the IRCnet-network to other servers. There's another server on IRCnet called The 2 servers (dinf and krameria) can reach each other trough the IRCnet. But sometimes it happens that a router between these two computers falls down. In the IRC community they name this phenoneme: netsplit. The two servers will function, without each other. So in this way, 2 different networks are formed from 1 network. After the split, the 2 networks are synchronized and they function back as one network.

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