Introduction

Domain Name System

The real Internet addresses, for locating web servers are 32-bit numbers. Domain names are used because they are easier to remember and key in than numbers, and because they provide a more permanent identifier than an IP address. A critical part of the Domain Name System (DNS) is to translate a domain name into its associated numerical address. That process is known as domain name resolution and is performed by name servers.

Distributed coordination is the key to the functionality of the DNS. The relevant IETF RFC describes domains as "a region of jurisdiction for name assignment, and of responsibility for name-- to-address translation." At the top of the hierarchy there is a "root server" (generally called "the root" or "the dot"). The root stores an authoritative list of top-level domains (TLDs), which will be explained in detail below. For each TLD, the root provides pointers to name servers that contain authoritative lists (often called "zone files") of second-level domain names-bcr, microsoft, syracuse, etc.-registered under that TLD. For each second-level name there is a pointer to two or more name servers that can resolve the name, and so on down the hierarchy.

Figure 1 illustrates the domain name hierarchy as of mid-1999:

              Figure 1: Domain Name Hierarchy

 The root is divided into 250 top-level domains (TLDs), and there are two broad classes of TLDs. So-called country-codes (ccTLDs) are two-letter codes based on the ISO3166 list of recognized countries. The six generic identifiers (gTLDs), most notably .com, are based on a simple taxonomy invented in the mid-1980s by the Internet's pioneers. The distinction between ccTLDs and gTLDs is based entirely on semantics and policy. It has nothing to do with how either operates. Both are simply text entries in a database with pointers to name servers.

Restrictions on the number and type of TLDs also are administrative, not technical. At some point, expanding the number of TLDs might break down the hierarchical nature of the DNS and create routing and congestion problems. But there are no serious technical objections to the addition of, for example, 5,000 new TLDs. Indeed, there are strong reasons to believe the system could accommodate as many as one million new TLDs. Whether there is any need or market demand for that many is an entirely separate matter.