How DNS Works

As a systems administrator, you might be responsible for connecting your company to the Internet, possibly for the first time. But before you can do so, you need to understand the basics of Internet addressing, find out how to get a Web address, and learn how to register your domain name. In "A DNS Primer," January 2000, Mark Minasi introduced the fundamentals of DNS. I will help you find resources for obtaining IP numbers and registering your domain name and provide practical information about DNS and its associated tools. (For more information about domain naming, see "Related Articles in Previous Issues," page 115.)

Obtaining Names and Addresses
The Internet Assigned Number Authority (IANA—http://www.iana.org) was historically the organization that doled out names and addresses. However, in 1993, the National Science Foundation (NSF) awarded to Network Solutions Incorporated (NSI—http://www.networksolutions.com) a 5-year contract that authorized NSI to operate the InterNIC (http://www.internic.net) name-registration service. At that time, InterNIC was the place that most people went to for a network identifier (NET_ID) or a domain name in the .com, .org, or .net namespace. When InterNIC assigned your IP address, you owned the address and could keep it even if you changed ISPs. Today, the process of obtaining addresses and of registering domain names has changed.

IP addressing in the modern era. InterNIC no longer assigns IP addresses and hasn't done so since about 1997. In the Western Hemisphere and some parts of the African continent, the American Registry for Internet Numbers (ARIN—http://www.arin.net) is now the IP number authority. The size of Internet routing tables was becoming unmanageable, so several years ago ARIN organized unused address space into Classless Inter-Domain Routing (CIDR) address blocks. ARIN assigns the CIDR address blocks, in turn, to ISPs or other regional number authorities, such as Réseaux IP Européens (RIPE—http://www.ripe.net) in Europe and the Asia Pacific Network Information Centre (APNIC—http://www.apnic.net). If you're in the United States, you need to coordinate with your ISP to obtain an IP NET_ID address from ARIN.

More than 50 percent of all possible Class C addresses are still available, but because the address supply is rapidly diminishing, getting a full Class C address is difficult. Instead, ISPs and ARIN distribute small address blocks and assign NET_IDs with as few as eight host addresses. IP number authorities assign the address blocks by using variable-length subnet masks (VLSMs), which provide a method of implementing classless addressing. A full description of VLSMs is beyond the scope of this article, but the sidebar "Subnetting and Variable-Length Subnet Masks" describes the motivation for using VLSMs and gives a VLSM example.

How can a company with several hundred systems operate with a mere handful of IP addresses? Companies might have many users, but generally they have only a few servers that require publicly visible IP addresses. Increasingly, organizations use private IP addressing internally and assign a public IP address to public servers. Companies might use the Internet Engineering Task Force (IETF) Request for Comments (RFC) 1918 private addresses or Network Address Translation (NAT) to statically map the server's public address to an internal private address. When a client system communicates with the Internet, NAT dynamically and temporarily assigns the client a public address. If the number of clients that need addresses exceeds the number of available public addresses, Port Address Translation (PAT—aka NAT overload) provides the addresses. NAT and PAT address management occur transparently at the router or NAT server. To learn more about current address assignments, visit John Crossley's IP Network Index (http://www.ipindex.net/), which cross-references IP Class A, B, and C network numbers with network names.

Registering domain names in the Internet's commercial era. The NSF's contract with NSI expired in April 1998. Unfortunately, no one planned how to handle domain name requests after that date, so NSF has extended NSI's contract several times, most recently through September 2000. In 1998, the Internet Corporation for Assigned Names and Numbers (ICANN—http://www.icann.org) began creating a fair and efficient domain-name-registration system. Many factors influenced the formation of ICANN, and you can find a good description of its history and evolution at the National Telecommunications and Information Administration (NTIA) Web site (http://www.ntia.doc.gov/ntiahome/domainname). Although NSI remains the sole administrator of names in the .com, .org, and .net namespace, many companies, such as America Online (http://www.aol.com), Internet Names WorldWide (http://www.internetnamesww.com), and register.com (http://www.register.com), can register names in that space. You can find a list of all accredited name registrars at the ICANN Web site.

DNS Internals
DNS is a distributed database that contains host, mail server, name server, and other domain information. You must maintain a primary name server and at least one secondary name server for every Internet domain. When a client system on the Internet needs to find a server's IP address (e.g., a reader of this article surfing to http://www.win2000mag.com or sending email to gkessler@symquest.com), the client sends a DNS query to its local name server. If the local name server doesn't have the necessary target server address information, the local name server sends a query to one of 13 well-known root name servers on the Internet. The query then proceeds to one of the target domain's name servers for final resolution. Many ISPs provide primary and secondary DNS service; others provide only secondary DNS service and require the customer to host the primary name server.

Examining DNS file structure will help you understand the name-lookup process. DNS information resides in simple text files called zone files, which contain information called Resource Records (RRs). The most common RRs are

• Start of Authority (SOA)—denotes the primary name server for a domain and a few additional administrative items
• Address (A)—supplies a host name's IP address
• Canonical Name (CNAME)—provides alias host names so that you can associate more than one host name with an IP address
• Pointer (PTR)—associates a host name with an IP address and performs reverse name lookups
• Mail Exchanger (MX)—defines a domain's mail systems
• Name Server (NS)—defines a domain's name servers