What Is a DNS Table?

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The Domain Name System serves as the "white pages" of the Internet, containing the IP addresses required to connect to Web pages, FTP servers and other computers connected to the Internet. DNS servers maintain a table of sites and their corresponding addresses, providing that information to users as a navigational aid. Getting around the Internet without access to a DNS table is possible, but is not a very user-friendly experience.


IP Addresses

An IP address is a string of four numbers, each between 0 and 255. IP addresses are hierarchical in nature, with each succeeding number referring to a finer division of Web space. For example, many home routers use the 192.168.1.x address block, allowing up to 255 individual addresses within that space. If you know the IP address of a website or other system you wish to access, you can type that string of numbers into your browser and connect directly to the target system.

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Since memorizing large strings of numbers can be difficult and frustrating, the Domain Name System allows the use of easy-to-remember site names instead. DNS tables are lists of site names with the corresponding IPs attached, allowing browsers to search for target sites and connect to the appropriate servers. To speed queries, DNS servers cooperate with one another, handing off requests to appropriate name servers to prevent any one database from growing too large. Since the Internet can quickly route requests through many different nodes, this allows users to access the distributed directory of site names quickly instead of forcing them to wait through a long database search.


How Tables Work

If you type the address "maps.google.com" into your browser, it sends a request to your ISP's DNS server, which looks in its top-level DNS table and locates an active name server for .com addresses. It then passes the request to that server, which uses its own table to locate a name server for google.com addresses. The google.com name server then consults its table and provides the IP address for maps.google.com, and your browser connects to the site. Though your request may be routed through several different servers before you are directed to the proper address, the process may take less than a second.



Dynamic DNS

Another type of DNS table offers service to users who want to run a server from their home PC, but lack a permanent connection to the Internet. Since the IP address of a cable or DSL user may change from session to session, dynamic DNS servers offer those users the ability to correlate a permanent domain name address with their connection. These services use a program that can detect any change in your computer's IP address, transferring that information to the dynamic DNS server, which then updates its information to reflect the change. Dynamic DNS tables update much more frequently than traditional DNS tables, which can take hours or even days to reflect changes.




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