What is a Router?
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updated 8/2003

Routers, by definition, route data from one network to another. Therefore, the Internet would come to a halt without routers. The routers used to route Internet traffic are a lot more powerful and sophisticated than the ones used in the homes and small businesses, but the idea remains the same. The main difference is scale. Internet routers have to handle huge amounts of traffic. Therefore, they have to be "smarter."

You may have heard the word NAT associated with routers. NAT stands for Network Address Translation. A NAT router has two ends. One end interacts with the DSL/cable modem and obtains whatever public IP address (simply stated as IP from now on) the modem (or to be more precise, the ISP) gives it, regardless of whether this IP is always the same (static) or different each time a user dials into the ISP (dynamic). The router then translates that public IP to at least one private IP, usually one starts with 192.168. This private IP is given to a computer that is connected to the router at the other end. This computer therefore can have access to the Internet through the router.

Why go through all this hassle of translation? Well, it's all about a lack of available IP addresses and security. Hackers outside your home network cannot, at least not directly, access this computer because they can only access the router. Private IPs are non-routable, meaning the computers with them cannot transmit to and receive data from the Internet without having their IPs re-translated to the assigned public routable IP by the router. Therefore, think of the router as a doorman where all delivery packages have to go through. The FedEx person never comes into contact with the residents behind the door. Needless to say, it is imperative to keep the password to the router secret. In fact, if you could modify the default settings of the router, do so. This way, hackers have to guess what type of a router it is before they can start hacking. This is even more evidently so with routers that are also wireless access points.

Ok, now some routers also act as DHCP servers. (What does DHCP stand for is not important in this article because its full name, unlike that of NAT, does not provide an easy clue for its purpose.) DHCP is a piece of software that runs in the router that the router uses to give out any IP address a client computer connected to it wants. Remember, any computer that wants to access the Internet or connect to another computer within your local network has to have an IP, public or private.

You can have multiple computers connected to a router through the router's various Ethernet ports. In this setup, all these computers can share that single public IP, and therefore the DSL connection! The router keeps what is called a routing table in its flash-based memory. This table has basically 2 entries: the globally unique MAC address of the network card used by the computer and the private IP given to it by the DHCP server of the router. Through this routing table, the router knows what computer requests what data from the Internet and relays the incoming requested data to that computer.

Routers are a great invention in networking. The fact that you can get one for about 30 dollars is amazing to me. Of course, there are a lot more to routers than NAT and DHCP. There are things like firewall and port forwarding. But that is another story.

For more info on routers, check out these links:
http://snipurl.com/ah5o

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