The DNS Name Space

- Managing a large and constantly changing set of names is a nontrivial problem. The Internet is divided into over 200 top-level domains, where each domain covers many hosts. Each domain is partitioned into subdomains, and these are further partitioned, and so on.

- All these domains can be represented by a tree, as shown in Fig. The leaves of the tree represent domains that have no subdomains. A leaf domain may contain a single host, or it may represent a company and contain thousands of hosts.

                         Figure. A portion of the Internet domain name space. 

                                                         
                                       
- The top-level domains come in two flavors: generic and countries. The original generic domains were com (commercial), edu (educational institutions), gov (the U.S. Federal Government), int (certain international organizations), mil (the U.S. armed forces), net (network providers), and org (nonprofit organizations).

- In November 2000, ICANN approved four new, general-purpose, top-level domains, namely, biz (businesses), info (information), name (people's names), and pro (professions). In addition, three more specialized top-level domains were introduced at the request of certain industries. These are aero (aerospace industry), coop (co-operatives), and museum (museums).

- Each domain is named by the path upward from it to the (unnamed) root. The components are separated by periods (pronounced ''dot''). Thus, the engineering department at Sun Microsystems might be eng.sun.com., rather than a UNIX-style name such as /com/sun/eng.

- Domain names can be either absolute or relative. An absolute domain name always ends with a period (e.g., eng.sun.com.), whereas a relative one does not. Relative names have to be interpreted in some context to uniquely determine their true meaning.

- Domain names are case insensitive, so edu, Edu, and EDU mean the same thing. Component names can be up to 63 characters long, and full path names must not exceed 255 characters.

- Each domain controls how it allocates the domains under it. For example, Japan has domains ac.jp and co.jp that mirror edu and com. The Netherlands does not make this distinction and puts all organizations directly under nl.

- Thus, all three of the following are university computer science departments:

1. cs.yale.edu (Yale University, in the United States)

2. cs.vu.nl (Vrije Universiteit, in The Netherlands)

3. cs.keio.ac.jp (Keio University, in Japan)

- To create a new domain, permission is required of the domain in which it will be included. Similarly, if a new university is chartered, say, the University of Northern South Dakota, it must ask the manager of the edu domain to assign it unsd.edu.

- In this way, name conflicts are avoided and each domain can keep track of all its subdomains. Once a new domain has been created and registered, it can create subdomains, such as cs.unsd.edu, without getting permission from anybody higher up the tree.