Sams Teach Yourself TCP/IP in 24 Hours (4th Edition)

A name resolution problem occurs when a hostname to which a message is addressed cannot be resolved on the network. A name resolution problem is (arguably) not a connectivity problem, because it doesn't necessarily mean that the source computer cannot connect with the target. In fact, as was mentioned in an earlier section, the most common symptom of a name resolution problem is that the source computer can reach the target by IP address but can't reach the target hostname. Even though a name resolution problem isn't a connectivity problem in the strictest sense, as a practical matter, resources on today's networks are referenced by hostname or NetBIOS name, and your first attempt to connect to a host will probably be by name. If that attempt fails, you can begin the problem-solving steps discussed in the "Ping" section earlier in this hour. If you can still connect by IP address, you probably have a problem with name resolution.

Many common name resolution problems are obvious when you consider the process of name resolution (see Hour 11, "Name Resolution"). Some common causes are:

• The hosts file is missing or incorrect.

• The name server is offline.

• The name server is referenced incorrectly in the client configuration.

• The host you are trying to reach does not have an entry in the name server.

• The hostname used in the command is incorrect.

If you can't connect to a computer by hostname, try connecting to a different computer. If you can connect to Computer A by hostname and you can't connect to Computer B, chances are the problem has something to do with Computer B and how it is referenced by the name service. If you can't connect to either Computer A or Computer B, chances are the problem is a more general failure of the name service infrastructure.

By the Way

Keep in mind that your computer might also be resolving names by the hosts file (see Hour 11). If a DNS server fails, your computer will continue to resolve the names entered explicitly in the hosts file. The hosts file might therefore provide a clue about why some names are resolved and some are not.

If you are experiencing name resolution problems on a network that uses a name server, it is a good idea to ping the name server to make sure it is online. If the name server is beyond the local subnet, ping the gateway to ensure that name resolution requests can reach the name server. Double-check the name you entered to ensure that it is the correct name for the resource. If none of these measures lead you to a solution, you can use the nslookup utility to query the name server about specific entries. See Hour 11 for more on nslookup.

If you are working at a computer and you don't know its hostname, use the hostname command. hostname is a simple command available on most operating systems that returns the hostname of the local computer. There are no options or parameters to hostname. Simply enter the command hostname and view the one-word response.