Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Name Server Concepts and Operation

DNS Name Server Concepts and Operation 1 2 3 DNS Name Server Data Storage: Resource Records and Classes

The three major functions of a name system are creating a name space, performing name registration and providing name resolution services. We've seen earlier in this larger section on the Domain Name System that DNS uses a hierarchical tree structure for its name space, and also a hierarchical tree for name authorities and registration. I'm sure that, given this, you will have to struggle to contain your surprise when I tell you that name resolution is also oriented around the notion of a hierarchical structure.

The devices that are primarily charged with performing the functions required to enable name resolution are name servers. They are arranged in a hierarchy that is closely related to the authority structure of the name system. Just as the authority structure complements the name structure but is not exactly the same as it, the name server architecture complements both the authority structure and the name structure, but may be different in its actual composition from both.

In a large DNS implementation, information about domains is not centralized in a single database run by one authority. Instead, it is distributed across many different authorities that manage particular top-level domains (TLDs), second-level domains or lower-level subdomains. In the case of the global Internet, literally millions of different “authorities”, many of them responsible only for their own local domain space, participate cooperatively in running the DNS system.

With authority for registration distributed in this manner, this means that the information about domains is similarly spread amongst many entities, resulting in a distributed database. A key concept in DNS name resolution is that each entity that maintains responsibility for a part of the name space must also arrange to have that information stored on a DNS server. This is required so that the server can provide the information about that part of the name space when resolution is performed. As you can see, then, the existence of a structured hierarchy of authorities directly implies the need for a hierarchy of servers that store that hierarchical name information.

Each DNS zone of authority is required to have one or more DNS servers that are “in charge” of managing information about that zone. These servers are said to be authoritative for the zone. Storing information about the domains, subdomains and objects in the zone is done by recording the data in special resource records that are read from DNS master lists maintained by administrators. Servers then respond to requests for this information.

Since information in DNS is stored in a distributed form, there is no single server that has information about every domain in the system. As we'll see in the section on name resolvers, the process of resolution instead relies on the hierarchy of name servers described just above. At the top of the DNS hierarchy is the root domain, and so we also see there the root name servers. These are the most important servers, because they maintain information about the top-level domains within the root. They also have knowledge of the servers that can be used to resolve domains one level below them. Those servers in turn are responsible for the TLDs and can reference servers that are responsible for second-level domains. Thus, a DNS resolution may require that requests be sent to more than one server.

DNS Name Server Concepts and Operation 1 2 3 DNS Name Server Data Storage: Resource Records and Classes