IP Default Gateway
Previously you saw gateways as hardware that translates among different protocols. Typically, when used in this context, the hardware is simply called a gateway.
The term gateway means something slightly different in the world of TCP/IP and other routable protocols. Rather than translating from one protocol to another, a TCP/IP default gateway is the default exit door through which traffic destined for TCP/IP networks other than the local network is sent.
The default gateway operates at a lower level of the OSI model than do the application gateways discussed earlier in this text.
When traffic is exchanged among hosts on the local network only, it does not need to know anything about a default gateway. However, when network traffic has a destination outside of the LAN, it must have an exit door.
The client PC may tell you network unreachable or simply hang when you try to access those outside networks. If you can reach resources (like files) on a local server, and other client PCs in your area can reach those outside networks, check your default gateway setting. It may be missing or incorrect.
With Windows 9.x or NT 4 this is found in the TCP/IP settings. Sometimes you will be asked to set this manually on each client computer. Usually, it is automatically configured by the DHCP server (the TCP/IP service that assigns address information, which is discussed later in the chapter), along with the clients own IP address and name server address, when the client machine is booted.
It is possible in NT to select several default gateways so that if a route is down, NT will try alternate exit doors. This is a simple approach where, if sending the packet through one gateway times out, NT just goes down the list until it finds an exit.
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