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Windows NT TCP/IP Network Administration by Robert Bruce Thompson, Craig Hunt

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Checking Routing

The “network unreachable” error message clearly indicates a routing problem. If the problem is in the local host’s routing table, it is easy to detect and resolve. First, use netstat -nr or route print to see whether or not a valid route to your destination is installed in the routing table.

For example, a user reports that the “network is down” because he cannot FTP to ftp.microsoft.com, and a ping test returns the following results:

% ping ftp.microsoft.com
PING ftp.microsoft.com: 32 data bytes
sendto: Network is unreachable
ping: wrote ftp.microsoft.com 32 chars, ret=-1
sendto: Network is unreachable
ping: wrote ftp.microsoft.com 32 chars, ret=-1

Based on the “network unreachable” error message, check the user’s routing table. In our example, we’re looking for a route to ftp.microsoft.com. The IP address[37] of ftp.microsoft.com is 205.187.99.57, which is a class C address. Remember that routes are network oriented. So we check for a route to network 205.187.99.0. If a specific route is not found, remember to look for a default route. If netstat shows the correct specific route, or a valid default route, the problem is not in the routing table. In that case, use tracert, as described in the next section, to trace the route all the way to its destination.

If netstat doesn’t return the expected route, it’s a local routing problem. There are two ways to approach local routing problems, depending on whether the system uses static or dynamic routing. Most systems that ...

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