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The Domain Name System
Contributor: Pay As You Host

The Domain Name System is at the heart of the Internet, and understanding
how it functions is especially crucial if you manage a website. Below you'll find basic and detailed explanations about the world of DNS.

Basic Explanation


DNS is a system which maps your domain name to the actual computer it's hosted on. DNS converts your domain name into an address, which can be used to locate your site somewhere on the Internet, allowing you to move your website from one provider to another without changing your domain name, and allowing the host to move your site around within a particular host's network (i.e. from one server to another).

In order for other people to know where your site is located, hosts run a system which, on request, will return the address of a server (called an IP address) depending on the domain name supplied. This means that if the host wants to move your site from one server to another on the host's network, the host would first move the files from one computer to another, and then adjust the settings to point your domain name to a new address (the address for the new server).

The service, run by nearly every ISP or hosting company, which does the above (i.e. converts a domain name to a computer address) is called a DNS server or Name server.

When you move your website from one hosting company to another, the DNS server for your domain changes. This means that if, for example, you moved a domain away from an existing host, people would stop requesting DNS information from that DNS server, and begin requesting it from the DNS server of your new hosting company.

DNS Recursing

When someone wants to find out the address of your website, they would not normally connect directly to the host's DNS server to find the information, as this would require them to be able to find out which DNS server a domain name is hosted on. When someone wants to find a website, they will normally make a request to the DNS server of their ISP or connection provider. When they make a request from their ISP's DNS server, this server then in turn requests the information from the 'authoritative nameserver' for the domain. This is known as DNS recursing, and a single request of this sort is known as a recursive DNS lookup (recursive simply implies that the request was passed to more than one server).

In this way, someone accessing the Internet only need know about one DNS server - their ISP's server.

DNS Caching

In the above example (where someone requests details of a site from their DNS server, which in turn requests the details from the DNS server hosting the site), an ISP's DNS server will not pass on a request every time one of their customers wants to view a site, as this would cause massive amounts of extra Internet traffic, and would also mean that many DNS servers (especially ones hosting popular sites) would be inundated with requests.

Instead of passing on a request every time a DNS server receives one, it will remember the last address it was given for the domain name for a fixed period of time, and simply pass this response back to any customers who access the same site. This is known as caching.

Normally, a DNS server will not cache the address of a domain name for more than 24 hours. However, some may be less (say, once per hour or few hours), and, in some rare cases, slightly more. The period of time a DNS server will cache details is up to the administrator of that server.

Problems caused by DNS caching

An unfortunate effect of DNS caching is that if a site needs to be moved, it can take up to 24 hours before all other DNS servers re-check with the DNS server hosting the site. This means that there may be a period of time when some DNS servers have the new address, and some have the old address. As time passes, more people will be able to see the new site.

Detailed Explanation


Every domain name on the Internet has an IP address associated with it. This IP address in turn is associated with a computer. The process of converting from a domain name to an IP address is called a DNS lookup.

There are two types of DNS server. A forwarding DNS server and a full DNS server. A forwarding DNS server will take a request for a domain name, and simply pass it on to another DNS server (normally always the same server). A full DNS server can do a number of things, including recursive lookups, DNS hosting of local sites, and caching (see below for details of these).

Locally hosted sites

If a domain is hosted locally, normally it will be listed on the local DNS server. When someone makes a request for this domain to the local DNS server, it will simply respond with the associated IP address.

Recursive Lookups

If a request is made to a DNS server, and the domain requested is not hosted on that server, the DNS server will lookup the domain name by traversing a number of other servers.

Initially, the DNS server will connect to a root name server (of which there are approximately 10). The root name server will then examine the domain requested, and inform the DNS server of where it may be hosted. For example, with a domain, the root name server would suggest, and possibly some alternative servers.

The DNS server will then move onto one of the returned servers, and request the same domain from the new server. This server will in turn suggest another DNS server. This continues until the DNS server on which the site is hosted is found (known as an authoritative nameserver for the domain), and the IP address is returned. At this point, the IP address is passed back to the original client, and they then access the site.


When a recursive lookup is done, the DNS server will cache the supplied IP address for a fixed period of time (usually 24 hours). If any clients request that same domain name again in this time, rather than doing another lookup, the DNS server will report with the cached IP address. This prevents excessive load on DNS servers and bandwidth.

Caching problems

The downside to caching is that if a change needs to be made to a domain name, it can take up to 24 hours before all DNS servers' caches have cleared, and they are supplying the new IP address to their clients.

It is possible for a DNS server to request a timeout period other than 24 hours for specific domain names. This means that when a DNS server caches the details it should read this timeout and instead of caching the details for the full 24 hours, is will only wait the requested period. This can be useful if the details for a domain name are changed regularly, or if it is expected to be changed in the near future.

Avoid the Endless Wait

One piece of advice about changing Name Servers. Although your existing DNS company (Domain Name reseller) may 'partially transfer' your Domain Name, they may not configure the actual Name Server to point towards your new website host's Name Sever. You may otherwise have an endless wait as your Domain Name continues to point towards your existing server. Make doubly sure you do not need to configure the DNS yourself by discussing this issue with your existing Domain Name provider.

Contributor Information

Pay As You Host (PAYH) offers great value, high quality web hosting that features flexibility and control including: no enforced annual hosting contract; their own infrastructure; hardware and software customised in-house; automated account and hosting set-up; exclusive hosting control panel; and full support services (phone and email).

As part of the Metronet group, PAYH assures you're not dealing with a new company that might disappear overnight, but rather one that has been providing a strong portfolio of Internet Services to thousands of people since 1996.

To find out more about Pay As You Host visit their website at:, or email:

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