The first major step to begin the actual planning of your environment's Active Directory infrastructure is to determine your Active Directory DNS namespace. A namespace is an area that can be resolved. A phone book is a good example of a namespace. It is organized by city, a person's last and first name, and address; you can resolve that name to a phone number. DNS uses the namespace to resolve a fully qualified domain name to an IP address. Because Active Directory names are DNS names, you must carefully plan your DNS namespace implementation. This article shows you how to plan your Active Directory name in a manner that is effective and meets the needs of your organization.
Overview of the DNS Naming Hierarchy
Before planning your DNS namespace for the Active Directory, you need to make certain that you understand some fundamental DNS concepts. If you have worked with DNS in the past or have previously studied DNS, you can skip to the next section. If not, this section serves as a primer or a good review.
Domain Name System (DNS) is an industry standard for resolving hostnames to IP addresses. DNS is implemented in Windows 2000 as the naming standard of choice due to its wide usage, reliability, and extensibility. Every time you use the Internet, you are accessing Web sites using DNS. DNS enables us to use a friendly domain name (for example, http://www.microsoft.com) to contact a Web site instead of having to remember its IP address (for example, 126.96.36.199). DNS takes care of the resolution process. It is important to understand the DNS hierarchy because it is implemented in the Active Directory.
DNS names are made up of a hierarchy structure, consisting of a tree of unique domain names. Each domain contains information about that segment of the DNS namespace. The hierarchy is made up of the Internet root, first-level domains, second-level domains, child domains, grandchild domains, and so forth, as shown in Figure 1.
Figure 1 The DNS hierarchy.
The Internet root is represented by a ".", and it is the very top of the tree structure. The next level in the tree are first-level domains—such as com, net, edu, org, mil, gov, and so forth. The second-level domains provide the business or organization name in the hierarchy. Child and grandchild domains can exist down the tree for each second-level domain. Within the second-level, child, or grandchild domain, a particular "host" or computer can be contacted, as shown in Figure 2.
Figure 2 Resolution to a host computer.
When the resolution reaches a host, the name is called a Fully Qualified Domain Name (FQDN). It shows the full relationship of a host computer to the domain, as shown in Figure 3.
Figure 3 Fully Qualified Domain Name (FQDN).
A DNS zone is a unit established for administrative purposes that contains a discreet and contiguous portion of the namespace. Because a large domain can be difficult to administer, they are often broken into zones so that different administrators can control different zones. Each zone contains at least one name server that contains the portion of the DNS database for that zone. The name server holds the domain name to IP address mappings for that particular zone. One name server holds the primary zone database file, and can make updates or changes to that file. Other name servers in that zone can contain a copy of the primary zone database file—called a secondary zone database file (see Figure 4). The name server with the primary zone database file is referred to as "authoritative" because it can make changes. The name servers that hold the secondary zone database file cannot make changes to the zone database file, but the primary name server can send updates to them through a process known as zone transfer. This process ensures that all name servers within a zone have an exact copy of the zone database file and can resolve DNS requests from client computers.
Figure 4 Zones contain name servers that hold the zone database file for that zone.
How DNS Name Resolution Works
As mentioned previously, the purpose of DNS is to resolve a fully qualified domain name to an IP address. Computers in a TCP/IP network must have this IP address in order to communicate with the desired computer. DNS resolves FQDNs by using a forward lookup query. In a forward lookup query, the following process occurs:
A client computer makes a name resolution request to a DNS name server. For example, a client computer in the learnad.com domain requests a resolution for server1.knowad.com. If the name server has authority over that domain, it can return the request. If not, the name server queries a number of other servers to resolve the name. In this example, the name server in learnad.com does not have authority over knowad.com, so the name server sends a request to a DNS root server.
The DNS root server returns the address of a com name server, and the learnad.com name server queries that server for the resolution.
The com name server checks its database file for knowad. If it finds knowad, it returns the name server for knowad to learnad.
The name server in learnad.com queries the name server in knowad.com for the resolution. Then, knowad.com checks its database file and finds Server1. It then passes the IP address for server1.knowad.com back to the name server at learnad.com, which returns the IP address to the client.
The client now can establish a TCP/IP connection with server1 and communication with server1, and the requesting client can take place.
In most circumstances, DNS resolutions require a number of name servers that have authority over their particular domains or zones. This entire process is invisible to the user.