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Print Formattednetworking > technologies > routers

Routers

Routers operate at the IP layer and work with IP numbers and masks to provide connections between two different networks. Routers are commonly used to link up Wide Area Networks (WAN), but also handy on smaller systems as well. A Router can be thought of as a Layer 3 OSI model bridge, that is, it operates at the Network OSI layer. The two networks do not have to be running the same Protocol, for example either TCP/IP or Token Ring. The router can provide translation between the two protocols. Eg Internet Protocol (IP), Internet Packet Exchange (IPX), and AppleTalk. If two networks with different IP ranges are to be connected then a router can handle this problem. The Router is declared to be the Gateway node for each node in both networks. If a NIC has requested to communicate with another NIC outside the local area network, the data packets is simply referred to the gateway or router node and it passes the request out of the local area network.


A simple router can be built up using a computer with two NIC's in it, each one configured to work properly on the respective networks. The logic of the router program in the computer is to move the data where required between the two different networks (ie not segments of the same network as a bridge does).

A common use for routers is to provide Network Address Translation (NAT). This allows one IP number on one system e.g. LAN-A to be expanded to as many IP's in a different network e.g. LAN-B as required. This is particularly handy when a large LAN-B may have only have access to a couple of real Internet IP numbers but many nodes on the LAN-B to be connected. This commonly happens in schools that have come onto the Internet scene relatively late and real Internet IP numbers scarce and expensive. It means the Internet eg LAN-A, sees a lot of traffic coming from one of its nodes, the router, but inside LAN-B there maybe hundreds of NIC's with a node and individual IP number, and each directing their traffic where required out through the router. The multiple LAN-B nodes share a single IP alias on the LAN-A. The router therefore conceals the structure of the internal network and greatly decreases the risk of external attack.

Likewise firewall rules can be included in the Router program to enhance security further providing higher levels of filtering or blocking to ports and protocols in TCP/IP. For example, this means some internal computers may be allowed access through FTP but the majority will be denied access. Routers are very flexible, but are also potentially much more complex to set up if advanced functions are to be used. It simply means the level of security internally on the LAN can be lower, and still have a very high Internet security.

Routers are also able to communicate amongst themselves to keep data bases (routing tables) on how many hops through other nodes it may take to reach another network. Consequently it can determine what is the optimum route to connect to a given destination. It does this by analysing the packet header. This slows down the data transfer but greatly enhances flexibility.

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