Linux Internet Sharing

Internet Sharing using a Linux Box
By Amarjyoti Krishnan

If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.

-- George Bernard Shaw

The computer world was similar to all of us having an apple each till some time back. And then the wise men from the industry made networks. Ideas started flowing all over and soon came in the internet. The internet has been the best thing that has happened to the computer world so far. It has created a platform where we can share our ideas.

Since the Internet is a large network composed of smaller networks, it made sense to break the address space into smaller chunks. Network classes enable us to break down this address space. In IPv4 the various classes of networks are -

Class A networks have an address range between 1.0.0.1 to 126.255.255.254 and support 16 million hosts on each of 127 networks.
Class B networks have an address range between 128.1.0.1 to 191.255.255.254 and support 65,000 hosts on each of 16,000 networks.
Class C networks have an address range between 192.0.1.1 to 223.255.254.254 and support 254 hosts on each of 2 million networks.
Class D networks have an address range between 224.0.0.0 to 239.255.255.255 and are reserved for multicast groups.
Class E networks have an address range between 240.0.0.0 to 254.255.255.254 and are reserved for the future

Such a system makes things simple and networks are manageable. However, they cannot communicate with each other. The scenario is very similar to how communities grew around the world. With time people from different communities needed to communicate with each other, however language was the barrier. The solution they found was people who understand both the languages i.e. translators. So if you need to talk to a beautiful French girl, all you need to do is get somebody who understands French and can translate English to French and vice versa. Routers /gateways do the same in computer networks. Different networks communicate with each other using routers.