Basically DNS is what converts a name (i.e. bbc.co.uk) to an IP address. A unique number which identifies a computer on a network (i'm simplifying it a bit here)
There are whats known as 13 Root Level DNS servers in the world. There can only be 13 due to the way its designed. Basically every public DNS server (computer address book) gets information that it doesn't know about from these Daddies.
These dadies, don't actually hold the information, but they know where to go to get it.
e.g. you type "
www.xr219.co.uk" into your web browser.
Assuming you haven't been to xr219.co.uk before, your computer queries its DNS server...
It's more than likely that that DNS server doesn't know where xr219.co.uk is either, so it forwards the request on to its primary DNS server.
Now that one, may still not know the anser... BUT, it knows were the root servers are, so its says, I don't know, but i know the guy who does, so a the query is sent on to one of the 13 root dns servers.
The root the replies with.... "i don't know the anser... BUT I know the guy who does"
The Root then sends the
address of the DNS server who is authoritive for that zone (in this example, xr219.co.uk) (in this example the term zone and domain name are the same thing.)
Every DNS name has a DNS server which is authoritive for it. That basically means he's the master record keeper for that zone (xr219.co.uk) and what he says goes, in so far as that zone is concerned.
So the DNS server you first queried, gets the address of the DNS server where xr219.co.uk info can be found. and thus ultimately works out the address of xr219.co.uk and passess this to your computer.
Now the DNS servers aren't totally daft, and so the information they recieve linking xr219.co.uk to an IP address they keep hold of for a while, just incase you need to use it again.
The period that the information is kept for is know as the TTL (Time To Live)
So lets say an hour later, you've rebooted your PC and you think... I want to go back to XR219.co.uk. Your PC will query the first DNS server, which should have the record of your earlier lookup (assuming the TTL hasn't expired) and give your PC the answer back very quickly.
It should be noted that this answer is know as non authorititive, because your DNS server is not the authorititive server for xr219.co.uk... he's just got a copy of that address.
now if you can follow that your doing well!
So... if there is a problem with one or more of the root dns servers, computers take a good while longer to work out where other ones live.
