Frequently Asked Question

DNS Basics
Last Updated 7 years ago

DNS (Domain Name System) is simply a translation service that enables us to remember "names" rather than numbers. The Internet works on Numbers, and all internet (Version 4) addresses are numeric, 4 groups of numberic numbers to be precise. For example:

127.234.111.105

Now, I can remember several key IP Addresses, and if I had to, I could probably remember another 10 or so, but that"s it. So instead we use domain names, like bbc.co.uk or gen.net.uk which we can remember much easier. Domain names themselves are made up of parts, each seperated by a . for example:

www.bbc.co.uk is in fact :

uk. co. bbc. www (because we process them from right to left) the meaning is:

uk = the UK Registry or Nominet to give it its real name. This is the company that maintains the domain names in the uk namespace (that"s any domain ending in .uk).

co = this is a subdomain, its short for "company" and signifies a company domain.

bbc = this is the actual domain name, registered by the BBC.

www = this is a "record" within the bbc domain, in this case the www translates into an IP address for the server which hosts the BBC website.

Ok, so now we have the basics for how the DNS system works, and why we can type www.bbc.co.uk into our browsers and the BBC website comes up. We know it as bbc.co.uk and our computer knows it as something like 12.48.24.67

So, we can get an IP Address for any domain name, and I suggest you have a look over at http://www.gensupport.net in support library/diagnostic tools and check out the whois lookup at the bottom of the page. Select A records, and type in www.bbc.co.uk and you"ll get:

www.bbc.co.uk. 299 IN CNAME www.bbc.net.uk.
www.bbc.net.uk. 166 IN A 212.58.246.95

Ok, so that"s saying www.bbc.co.uk is actually www.bbc.net.uk. and that www.bbc.net.uk is at 212.58.246.95. Simples.

There is one more critical part of DNS that you should know about, Reverse DNS. Just as you would expect, for a given IP Address, reverse DNS can give you back its domain name. The internet standard 1 contains specification documents called RFC"s, and RFC1124 & RFC1124 deal with DNS, and stipulate that every host on the internet should have complete forward and reverse DNS. That is, ns0.gen.net.uk translates to 217.36.11.19 and 217.36.11.19 should translate to ns0.gen.net.uk. This is basic configuration and all ISP"s take this stuff seriously.

If you want to check our reverse DNS you can do it with the diagnostic tools above by selecting PTR Records and entering an IP Address.

To help with quick checking, there is a reverse DNS Diagnostic on the same page. Enter an IP Address and we will get its reverse translation, then check its forward translation is correct too.


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