Dear non-Experts,

The purpose of the DNS is to enable Internet applications and their users to name things that have to have a globally unique name. The obvious benefit is easily memorizable names for things like web pages and mailboxes, rather than long numbers or codes. Less obvious but equally important is the separation of the name of something from its location. Things can move to a totally different location in the network fully transparently, without changing their name. www.isoc.org can be on a computer in Virginia today and on another computer in Geneva tomorrow without anyone noticing.

In order to achieve this separation, names must be translated into other identifiers which the applications use to communicate via the appropriate Internet protocols. Let’s look at what happens when you send a mail message to me at daniel.karrenberg@ripe.net. A mail server trying to deliver the message has to find out where mail for mailboxes at ‘ripe.net’ has to be sent. This is when the DNS comes into play.

The mail server transmits this question, called a ‘query’ in DNS terminology to the DNS. Quickly it receives as answer:

ripe.net. 1800 IN MX 100 postman.ripe.net.
ripe.net. 1800 IN MX 150 postboy. ripe.net.

“Mail for ‘ripe.net’ should be sent first to the computer called ‘postman.ripe.net’. If ‘postman.ripe.net’ is not available, try ‘postboy.ripe.net’”.

The mail server will now make a connection with ‘postman.ripe.net’. In order to do this it needs to know the numeric Internet address for ‘postman.ripe.net’. This numeric address is sufficient to send Internet packets directly to the computer called ‘postman.ripe.net’ from anywhere in the Internet. Again the DNS is queried and it returns ’193.0.0.199′.

postman.ripe.net. 172800 IN A 193.0.0.199

From here on, the mail server delivering your message can directly communicate with the mail server that receives and stores my mail for me to collect later on.

It is obvious that the information we just obtained in our example cannot come from a central list somewhere. This list would simply be too large, change too often and its repository would be quickly overwhelmed with queries.