Name Resolution Problems

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Name Resolution Problems

If you have configured TCP/IP correctly and the protocol is installed and working, then the problem with connectivity is probably due to errors in resolving host names. When you test connectivity with TCP/IP addresses, you are testing a lower-level of connectivity than users generally use. When users want to connect to a network resource, such as mapping a drive to a server or connecting to a Web site, they usually refer to that server or Web site by its name rather than its TCP/IP address.

In fact, users do not usually know the IP address of particular server. The name used to establish a connection must be resolved down to an IP address so that the networking software can make a connection. After you’ve tested the IP connectivity, the next logical step is to check the resolution of a name down to its IP address. If a name cannot be resolved to its IP address or if it is resolved to the wrong address, users will not be able to connect to the network resource with that name, even if you can connect to it using an IP address.

Two types of computer names are used when communicating on the network. A NetBIOS name is assigned to a Microsoft computer, such as a Windows NT server or a Windows 95 client. A host name is assigned to non-Microsoft computer, such as a Unix server. (Host names can also be assigned to a Windows NT server running Internet Information Server. For example, the name www.microsoft.com refers to a Web server on the Microsoft Web site. This server is running on Windows NT.) In general, when using Microsoft networking, such as connecting to a server for file sharing, print sharing, or applications, you refer to that computer by its NetBIOS name. When executing a TCP/IP-specific command, such as FTP or using a Web browser, you refer to that computer by its host name.

A NetBIOS name is resolved to a TCP/IP address in several ways. Figure 16.29 shows how NetBIOS names are resolved. The TCP/IP client initiating a session first looks in its local name cache. If the client cannot find the name in a local cache, it queries a WINS server, if configured to be a WINS client. If the WINS server cannot resolve the name, the client tries a broadcast that only reaches the local subnet, because routers, by default, are not configured to forward broadcasts. If the client cannot find the name through a broadcast, it looks for any LMHOSTS or HOSTS files, if it has been configured to do so.

Finally, if the client cannot resolve a name in any other way, it queries a DNS server if it has been configured to be a DNS client. However, if the client specifies a name longer than 15 characters (the maximum length of a NetBIOS name), the client first queries DNS before trying a HOSTS file or WINS.

Host names are resolved in a similar manner. The client, however, checks sources that are used solely to resolve host names before trying sources that are using to resolve NetBIOS names. In resolving host names, the client first checks the HOSTS file, then the DNS server, if configured to be a DNS client. These two sources only resolve host names. If the client cannot resolve the name, it checks the WINS server, if configured as a WINS client, tries a broadcast, and then looks in the LMHOSTS file. The last three methods to resolve a name are used to resolve NetBIOS names, but it is possible for a host name to be listed in these sources.

Several tools are available to test name resolution. They are discussed in the following sections.

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