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Table Of Contents  CertiGuide to Network+
 9  Chapter 0101:  TCP/IP
      9  VI  TCP/IP Routing

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Static vs. Dynamic Routing
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VII  Ports
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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.

Let me out of here!

If no default gateway is set on a client system, and no static routes have been set up on that client, then traffic destined for hosts on other networks (like the Internet) cannot be sent out to those networks.


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 PC’s 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 client’s 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.

Default Gateway

The default gateway IP number is usually a router that accepts all network traffic not meant to live on the LAN. The default gateway must be setup for TCP/IP based data to be routed to its final destination. The default gateway is not necessary in a single segment (subnet).



Previous Topic/Section
Static vs. Dynamic Routing
Previous Page
Pages in Current Topic/Section
1
Next Page
VII  Ports
Next Topic/Section

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