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Lesson 1: Understanding DNS

DNS is most commonly associated with the Internet. However, private networks use DNS extensively to resolve computer names and to locate computers within their local networks and the Internet. DNS provides the following benefits:

• DNS names are user-friendly, which means that they are easier to remember than IP addresses.
• DNS names remain more constant than IP addresses. An IP address for a server can change, but the server name remains the same.
• DNS allows users to connect to local servers by using the same naming convention as the Internet.

NOTE

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For more information on DNS, see RFC 1034 and RFC 1035. A Request for Comment (RFC) is a published document on a standard, protocol, or other information pertaining to the operation of the Internet. To read the text of these RFCs, use your Web browser to search for "RFC 1034" and "RFC 1035."

After this lesson, you will be able to

• Explain the function of DNS and its components.

Estimated lesson time: 15 minutes

Domain Name Space

The domain name space is the naming scheme that provides the hierarchical structure for the DNS database. Each node represents a partition of the DNS
database. These nodes are referred to as domains.

The DNS database is indexed by name; therefore, each domain must have a name. As you add domains to the hierarchy, the name of the parent domain is appended to its child domain (called a subdomain). Consequently, a domain's name identifies its position in the hierarchy. For example, in Figure 8.1, the domain name

sales.microsoft.com

identifies the sales domain as a subdomain of the microsoft.com domain and microsoft as a subdomain of the com domain. The hierarchical structure of the domain name space consists of a root domain, top-level domains, second-level domains, and host names.

NOTE

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The term domain, in the context of DNS, is not related to domain as used
in Microsoft Windows 2000's directory services based on Active Directory technology. A Windows 2000 domain is a grouping of computers and devices that are administered as a unit.

Figure 8.1 The hierarchical structure of a domain name space

Root Domain

The root domain is at the top of the hierarchy and is represented as a period (.). The Internet root domain is managed by several organizations, including Network Solutions, Inc.

Top-Level Domains

Top-level domains are two- or three-character name codes. Top-level domains are arranged by organization type or geographic location. Table 8.1 provides some examples of top-level domain names.

Table 8.1 Top-Level Domains

Top-level domains can contain second-level domains and host names.

Second-Level Domains

Organizations, such as Network Solutions, Inc., assign and register second-level domains to individuals and organizations for the Internet. A second-level name has two name parts: a top-level name and a unique second-level name. Table 8.2 provides some examples of second-level domains.

Table 8.2 Second-Level Domains

Host Names

Host names refer to specific computers on the Internet or a private network.
For example, in Figure 8.1, Computer1 is a host name. A host name is the leftmost portion of a fully qualified domain name (FQDN), which describes
the exact position of a host within the domain hierarchy. In Figure 8.1, Computer1.sales.microsoft.com. (including the end period, which represents
the root domain) is an FQDN.

DNS uses a host's FQDN to resolve a name to an IP address.

NOTE

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The host name doesn't have to be the same as the computer name. By
default, TCP/IP setup uses the computer name for the host name, replacing illegal characters, such as the underscore (_), with a hyphen (-). For the accepted domain naming conventions, see RFC 1035.

Domain Naming Guidelines

When you create a domain name space, consider the following domain guidelines and standard naming conventions:

• Limit the number of domain levels. Typically, DNS host entries should be three or four levels down the DNS hierarchy and no more than five levels down the hierarchy. Increasing the number of levels increases the administrative taskload.
• Use unique names. Each subdomain must have a unique name within its parent domain to ensure that the name is unique throughout the DNS name space.
• Use simple names. Simple and precise domain names are easier for users to remember and enable users to search intuitively and locate Web sites or other computers on the Internet or an intranet.
• Avoid lengthy domain names. Domain names can be up to 63 characters, including the periods. The total length of an FQDN can't exceed 255 characters. Case-sensitive naming is not supported.
• Use standard DNS characters and Unicode characters.
• Windows 2000 supports the following standard DNS characters: A-Z, a-z, 0-9, and the hyphen (-), as defined in RFC 1035.
• The DNS Service also supports the Unicode character set. The Unicode character set includes additional characters not found in the American Standard Code for Information Interchange (ASCII) character set, which are required for languages such as French, German, and Spanish.

NOTE

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Use Unicode characters only if all servers running the DNS Service in your environment support Unicode. For more information on the Unicode character set, read RFC 2044 by searching for "RFC 2044" with your Web browser.

Zones

A zone represents a discrete portion of the domain name space. Zones provide a way to partition the domain name space into manageable sections.

• Multiple zones in a domain name space are used to distribute administrative tasks to different groups. For example, Figure 8.2 depicts the microsoft.com domain name space divided into two zones. The two zones allow one administrator to manage the microsoft and sales domains and another administrator to manage the development domain.
• A zone must encompass a contiguous domain name space. For example, in Figure 8.2, you couldn't create a zone that consists of only the sales.microsoft.com and development.microsoft.com domains, because these two domains are not contiguous.




Figure 8.2 A domain name space divided into zones

The name-to-IP address mappings for a zone are stored in the zone database file. Each zone is anchored to a specific domain, which is referred to as the zone's root domain. The zone database file doesn't necessarily contain information for all subdomains of the zone's root domain, only those subdomains within the zone.

In Figure 8.2, the root domain for Zone1 is microsoft.com, and its zone file contains the name-to-IP address mappings for the microsoft and sales domains. The root domain for Zone2 is development, and its zone file contains the name-to-IP address mappings for the development domain only. The zone file for Zone1 doesn't contain the name-to-IP address mappings for the development domain, although development is a subdomain of the microsoft domain.

Name Servers

A DNS name server stores the zone database file. Name servers can store data for one zone or multiple zones. A name server is said to have authority for the domain name space that the zone encompasses.

One name server contains the master zone database file, referred to as the primary zone database file, for the specified zone. As a result, there must be at least one name server for a zone. Changes to a zone, such as adding domains or hosts, are performed on the server that contains the primary zone database file.

Multiple name servers act as a backup to the name server containing the primary zone database file. Multiple name servers provide the following advantages:

• Perform zone transfers. The additional name servers obtain a copy of the zone database file from the name server that contains the primary database zone file. This is called a zone transfer. These name servers periodically query the name server containing the primary zone database file for updated zone data.
• Provide redundancy. If the name server containing the primary zone database file fails, the additional name servers can provide service.
• Improve access speed for remote locations. If a number of clients are in remote locations, use additional name servers to reduce query traffic across slow WAN links.
• Reduce the load on the name server containing the primary zone database file.

DNS is most commonly associated with the Internet. However, many private networks also use DNS to resolve computer names and to locate computers within their local networks and the Internet. In this lesson, you learned that some of the benefits that DNS provides include providing user-friendly DNS names that are less likely to change than IP addresses, and allowing users to connect to local servers by using the same naming convention as the Internet.

You also learned that the domain name space is the naming scheme that provides the hierarchical structure for the DNS database. The DNS database is indexed by name, so each domain (node) must have a name. The hierarchical structure of the domain name space consists of a root domain, top-level domains, second-level domains, and host names. Host names refer to specific computers on the Internet or a private network. A host name is the leftmost portion of a fully qualified
domain name (FQDN), which describes the exact position of a host within the domain hierarchy.

Finally, you learned about the following naming guidelines for domains: limit the number of domain levels, use unique names, and use simple names. Zones provide a way to divide the domain name space into smaller sections that represent
a discrete portion of the domain name space. You also learned that a DNS name server stores the zone database file, that the zone database file is replicated, and how to configure these zone transfers.