Name Resolution Methods

Internet host names are used because they are easier to remember than the long dotted decimal IP addresses. Host names are typically the name of a device that has a specific IP address, and on the Internet are part of what is known as a Fully Qualified Domain Name (FQDN). An FQDN consists of a host name and a domain name.

Although we have Social Security numbers and can remember them when we need to do so, life would be difficult if we had to remember the Social Security numbers of all our friends and associates. We might be able to remember the Social Security numbers of as many as 10 friends and relatives, but after that, things would get a bit difficult. Likewise, it’s easier to remember www.microsoft.com than it is to remember 198.105.232.6.

The process of finding the host name for any given IP address is known as name resolution, which can be performed in several ways: a HOSTS file, DNS, and WINS. But before you read about that, you need to first understand Internet domains and how they are organized.

Internet Domain Organization

On the Internet, domains are arranged in a hierarchical tree structure. The seven top-level domains currently in use are:

com: A commercial organization. Most companies will end up as part of this domain.

edu: An educational establishment, such as a university.

gov: A branch of the U.S. government.

int: An international organization, such as NATO or the United Nations.

mil: A branch of the U.S. military.

net: A network organization.

org: A nonprofit organization.

Warning 

Unfortunately, the word domain is used in several ways, depending on the context. In talking about the Internet, a domain refers to a collection of network host computers. See Chapter 4, “TCP/IP Utilities,” for a discussion of how Microsoft Windows NT Server defines a domain.

Your local ISP is probably a member of the .net domain, and your company is probably part of the .com domain. The .gov and .mil domains are reserved strictly for use by the government and the military within the United States. In other parts of the world, the final part of a domain name represents the country in which the server is located (.ca for Canada, .jp for Japan, .uk for Great Britain, and .ru for Russia, for example). Well over 130 countries are represented on the Internet. The .com domain is by far the largest, followed by the .edu domain.

If you want to contact someone within one of these domains by e-mail, you just add that person’s e-mail name to their domain name, separated by an at (@) sign. For example, if you want to e-mail the president of the United States, send your e-mail to this address:

[e-mail] 

To increase the number of domain names available for use—after all, there is only one mcdonalds.com domain name available—several alternative toplevel domains have been suggested. These include .firm for businesses and companies, .store for businesses selling goods rather than services, .arts for cultural and entertainment organizations, and .info for informational services.

InterNIC assigns all Internet domain names and makes sure that a name is not duplicated. Names are assigned on a first come, first served basis, but if you try to register a name that infringes on someone else’s registered trademark, your use of that name will be rescinded if the trademark holder objects.

Using HOSTS

Several automatic conversion systems are available to translate an IP address into a host name, and HOSTS is one of the simplest. You create a file called HOSTS and enter a line in the file for every system. Here’s an example:

198.34.56.25 myserver.com #My server's information  198.34.57.03 yourserver.com 

Now comes the nasty part. You must store this ASCII file on every single workstation on your network; when you make a change, you must change the contents of the HOSTS file on every single workstation on your network. This is a simple but painful process inside a network. But what happens if you want to go outside of this network to other networks or to the Internet? The file size would be simply enormous. Fortunately, there are better solutions, as you will see in the next two sections.

Note 

Any information entered to the right of a pound sign in a HOSTS file is ignored, so you can use this space for comments.

Using DNS

The abbreviation DNS stands for Domain Name Service. You use DNS to translate host names and domain names to IP addresses, and vice versa, by means of a standardized lookup table that the network administrator defines and configures. The system works just like a giant telephone directory.

Suppose you are using your browser to surf the Web, and you enter the URL http://www.microsoft.com to go to the Microsoft home page. Your web browser then asks the TCP/IP protocol to ask the DNS server for the IP address of www.microsoft.com. When your web browser receives this address, it connects to the Microsoft web server and downloads the home page. DNS is an essential part of any TCP/IP network, because it simplifies the task of remembering addresses; all you have to do is simply remember the host name and domain name.

DNS tables are composed of records. Each record is composed of a host name, a record type, and an address. There are several record types, including the address record, the mail exchange record, and the CNAME record.

The address record, commonly known as the A record, maps a host name to an IP address. The example below shows the address record for a host called mail in the company.com domain:

mail.company.com.     IN     A     204.176.47.9

The mail exchange (MX) record points to the mail exchanger for a particular host. DNS is structured so that you can actually specify several mail exchangers for one host. This feature provides a higher probability that e-mail will actually arrive at its intended destination. The mail exchangers are listed in order in the record, with a priority code that indicates the order in which the mail exchangers should be accessed by other mail delivery systems.

If the first priority doesn’t respond in a given amount of time, the mail delivery system tries the second one, and so on. Here are some sample mail exchange records:

hostname.company.com.  IN    MX    10 mail.company.com. hostname.company.com.  IN    MX    20 mail2.company.com. hostname.company.com.  IN    MX    30 mail3.company.com.

In this example, if the first mail exchanger, mail.company.com, does not respond, the second one, mail2.company.com, is tried, and so on.

The CNAME record, or canonical name record, is also commonly known as the alias record and allows hosts to have more than one name. For example, your web server has the host name www, and you want that machine to also have the name ftp so that users can use FTP to easily manage web pages. You can accomplish this with a CNAME record. Assuming that you already have an address record established for the host name www, a CNAME record that adds ftp as a host name would look something like this:

www.company.com.       IN    A      204.176.47.2 ftp.company.com.       IN    CNAME  www.company.com.

When you put all these record types together in a file, it’s called a DNS table, and it might look like this:

mail.company.com.      IN    A      204.176.47.9 mail2.company.com.     IN    A      204.176.47.21 mail3.company.com.     IN    A      204.176.47.89 yourhost.company.com.  IN    MX     10 mail.company.com. yourhost.company.com.  IN    MX     20 mail2.company.com. yourhost.company.com.  IN    MX     30 mail3.company.com. www.company.com.       IN    A      204.176.47.2 ftp.company.com.       IN    CNAME  www.company.com.

Note 

You can establish other types of records for specific purposes, but we won’t go into those in this book. DNS can become very complex very quickly, and entire books are dedicated to the DNS system.

Using WINS

WINS, or Windows Internet Naming Service, is an essential part of the Microsoft networking topology. But before we get into the discussion of WINS, we must define a few terms, including these two protocols—NetBIOS and NetBEUI.

NetBIOS  (pronounced net-bye-os) is an acronym formed from network basic input/output system, a Session layer network protocol originally developed by IBM and Sytek to manage data exchange and network access. NetBIOS provides an API (application programming interface) with a consistent set of commands for requesting lower-level network services to transmit information from node to node, thus separating the applications from the underlying network operating system. Many vendors provide either their own version of NetBIOS or an emulation of its communications services in their products.

NetBEUI  (pronounced net-boo-ee) is an acronym formed from Net-BIOS Extended User Interface, an implementation and extension of IBM’s NetBIOS transport protocol from Microsoft. NetBEUI communicates with the network through Microsoft’s NDIS (Network Driver Interface Specification). Today, NetBEUI is shipped with all versions of Microsoft’s operating systems and is generally considered to have a lot of overhead. NetBEUI also has no networking layer and therefore no routing capability, which means that it is suitable only for small networks; you cannot build internetworks with NetBEUI, and so it is often replaced with TCP/IP. Microsoft has added extensions to NetBEUI in Windows NT to remove the limitation of 254 sessions per node; this extended version of NetBEUI is called the NetBIOS Frame (NBF).

WINS is used in conjunction with TCP/IP and maps NetBIOS names to IP addresses. For example, you have a print server on your LAN that you have come to know as PrintServer1. In the past, to print to that server you needed only to remember its name and to select that name from a list. However, TCP/IP is a completely different protocol and doesn’t understand NetBIOS names; it therefore has no way of knowing the location of those servers or their addresses. That’s where WINS comes in.

Each time you access a network resource on a Windows NT network using TCP/IP, your system needs to know the host name or IP address. If WINS is installed, you can continue using the NetBIOS names that you have previously used to access the resources, because WINS provides the crossreference from name to address for you.

Note 

A NetBIOS name doesn’t always refer to just a machine. Several services on a machine can also have their own NetBIOS names.

When you install and configure TCP/IP, as described later in this chapter, you’ll see a place to specify the WINS server addresses. These addresses are stored with the configuration, and TCP/IP uses them to query for host names and addresses when necessary. WINS is similar to DNS in that it cross-references host names to addresses; however, as mentioned earlier, WINS references NetBIOS names to IP addresses, but DNS references TCP/IP host names to IP addresses.

Another major difference between WINS and DNS is that WINS builds its own reference tables dynamically, but you have to configure DNS manually. When a workstation running TCP/IP is booted and attached to the network, it uses the WINS address settings in the TCP/IP configuration to communicate with the WINS server. The workstation gives the WINS server various pieces of information about itself such as the NetBIOS host name, the actual username logged on to the workstation, and the workstation’s IP address. WINS stores this information for use on the network and periodically refreshes it to maintain accuracy.

Microsoft, however, has developed a new DNS record—called DNS Server—that allows the DNS server to work in perfect harmony with a WINS server. The Microsoft DNS Server software currently ships with Windows NT. Here’s how it works: When a DNS query returns a WINS record, the DNS server then asks the WINS server for the host name address. Thus, you need not build complex DNS tables to establish and configure name resolution on your server; Microsoft DNS relies entirely on WINS to tell it the addresses it needs to resolve. And because WINS builds its tables automatically, you don’t have to edit the DNS tables when addresses change; WINS takes care of this for you.

Note 

Network Address Translation (NAT) is the process of converting between the IP addresses used on a corporate intranet or other private network and Internet IP addresses. This process makes it possible to use a large number of addresses within the private network without depleting the limited number of available numeric IP addresses. NAT is usually performed within a router.

You can use both WINS and DNS on your network, or you can use one without the other. Your choice is determined by whether your network is connected to the Internet and whether your host addresses are dynamically assigned. When you are connected to the Internet, you must use DNS to resolve host names and addresses, because TCP/IP depends on DNS service for address resolution.