Q. I'm setting up a network lab at home in order to get some hands-on training and to prepare myself for my upcoming network exams. However, I'm having trouble getting the machines to communicate with one another. I'm using TCP/IP and I can access some computers from the server, but I have trouble accessing the server from those same machines. Some can join the domain, while others can't. Even when I get them on the domain, I have problems getting multiple servers to become domain controllers on the network. What's going on?
A: The problem is with identifying computers on your network. Generally, computers on TCP/IP networks are recognized by more than their IP addresses. A descriptive name is also used. The process of matching this descriptive name to the IP address of a particular machine is known as name resolution. There are two primary means of name resolution: NetBIOS and host name resolution.
Name Resolution Methods
Microsoft's original name resolution method, included in versions of Win9X, WinME, and WinNT4, was NetBIOS. NetBIOS names are made of sixteen characters. The first fifteen are generally chosen by a system administrator during setup as the computer name, while the sixteenth is reserved by the operating system to define what services are being offered by the machine.
Windows Internet Name Service (WINS) servers are used to dynamically map a unique NetBIOS name (which is unique) to the appropriate IP address, maintaining a flat database of these entries. If a WINS server is not available, a static file (LMHOSTS) can be created to manually create the resolution database. If neither of the previous options provide proper mapping, then Windows-based machines are capable of resolving NetBIOS through broadcasts across their local network segment. Once a system resolves a NetBIOS name, it stores this information in its NetBIOS name cache for faster resolution for future requests.
Microsoft's more recent offerings, those of the Windows 2000 and Windows XP families, utilize host names as part of a fully qualified domain name (FQDN) to resolve IP addresses. The FQDN consists of a host name, followed by domain identification. You've most likely observed these names using Internet URLs (i.e. ), where www identifies a computer on the microsoft.com domain. In Windows 2000/XP, the host name is the same as the computer name. On older operating systems, the host name can be entered during TCP/IP configuration - but it is recommended that the host name match the computer name, to reduce confusion.
Domain Name System (DNS) servers are used to store the host name to IP mappings. These mappings are distributed, meaning several servers can maintain portions of the entire database. Servers that maintain the database for a particular domain are known as authoritative for that domain. Authoritative servers handle requests for all mappings in their domain. Requests for resolution outside that domain are passed to DNS servers that are authoritative at higher levels (i.e. microsoft.com would pass requests for apple.com to a server that is authoritative at the .com domain level). Since these mappings exist in a hierarchical structure, computer names only have to be unique at their own network level. As with WINS, a static file can be created on the local machine that maintains the database of host name to IP mappings. This is the HOSTS file.
Name Resolution Order
When attempting to locate computer resources, Microsoft's server operating systems will use both name resolution methods. Descriptive names are treated as host names (DNS) first, especially if they are longer than fifteen characters, or contain a period (which NetBIOS does not allow). The following order is the default:
- Check to see if the name is the system's own host name.
- Check entries in the local HOSTS file.
- Query a DNS server.
- Switch to NetBIOS and check the NetBIOS cache.
- Query the WINS server.
- Broadcast for NetBIOS name resolution.
- Check the local LMHOSTS file.
If the descriptive name is considered to be NetBIOS from the beginning, the process is as follows:
- Check the NetBIOS name cache.
- Query the WINS server.
- Broadcast for NetBIOS name resolution.
- Check the local LMHOSTS file.
- Attempt host name resolution methods.
To specifically address the question above, be aware of the name resolution methods required by your network configuration. If there are NetBIOS-dependent clients involved, then WINS services or a LMHOSTS file are needed. If all clients are DNS dependent, then proper DNS mappings are required. All DNS clients must be configured to locate at least one DNS server to resolve DNS requests, or have a hard-coded HOSTS file in place.
As long as user names and passwords are consistent across the network, all the shares and resources should be readily available.
Shane Hicks (A+, Network+, MCSA MCSE on NT/W2K/XP, CTT+, MCT) is an independent consultant and technical trainer, providing support to individuals and small businesses. He's been "in the industry "for over 10 years. He gets e-mail at shane@absolutepowercomputing.com.
