My job requires me to spend most of my time moving around Dubai, meeting clients, proposing solutions to their problems or just simply fixing their servers when all has gone pair shaped. Naturally, having only lived here for 9 months of the year I still manage to get lost quite often and most times have no idea where the next client’s office is, what their numbers are etc so I have to rely on other sources for this information.

Luckily for me, we have an online yellow pages directory which is pretty comprehensive and has saved me many a day. We also have a dial in service where we can call in and request the number for a certain company or individual then call them up and ask for directions, again, a well used service.

Of course, without these services there would be other ways for me to obtain the information, but it would require a lot more time and effort on my part resulting in a loss of productivity, not only for myself but for those significant others around me I would constantly be bombarding for details.

In a sense, this is what a DNS server is to your network or the internet, an online directory which maps names and services to addresses.

Let me explain in a little more detail, say for example, I am trying to call Microsoft Licensing, but don’t have their number. The first thing I do is consult my online directory, be it the yellow pages if I have an internet access or the dial in service if I don’t. I give them the name of the company I wish to contact and they look it up in their database then get back to me with the correct contact number (or sometimes not). Armed with the number I can then contact Microsoft myself and have the conversation I need to with them.

DNS does precisely this for computers and networks. When you open internet explorer and try to connect to www.google.com, what actually happens is the following:

1.       Your computer asks its DNS server what the IP Address for www.google.com is

2.       The DNS server checks its local database for an answer and sends it back to your computer

3.       Your computer then contacts www.google.com directly on the IP Address it was given by DNS and retrieves the relevant information to display to you.

The same process takes place to deliver your email. You send the message to the email server, the email server queries DNS for a Mail Exchanger (MX) Record for the domain to which you are sending, and once it has received the IP Address for the remote domain, it sends the email to the recipients server.

As in the above examples, there are many services out there which are published using DNS for example:  Active Directory Services, SIP services (instant messenger servers) and web servers amongst others.

A DNS server basically consists or 2 types of Zone, a Forward Lookup Zone and a Reverse Lookup Zone.

A Forward Lookup Zone is used to store all information relevant to a particular domain based on that domain name, i.e. all information for the SortedIT.net domain is stored in the corresponding forward lookup zone of the authoritative DNS Servers. The only requirement for records to exist in this zone is that they have the FQDN attached to the end of them, e.g. blog.sortedit.net. The IP address of the host server for the forward lookup record can be anything.

Essentially, what a forward lookup zone does is map a user friendly FQDN to the IP address of a server, e.g. blog.sortedit.net = 64.202.189.158. The forward lookup zone for SortedIT.net will hold all the records for the servers and services which are hosted by my domain regardless of the IP address of the server on which they are hosted. Forward lookup zones CANNOT hold records for other domains, as in, I cannot create a record for mail.sorted.co.zw in the sortedit.net forward lookup zone.

The Reverse Lookup Zone, is exactly the opposite of the Forward Lookup Zone, it maps IP addresses to host names. Reverse Lookup Zones are bound to specific IP Subnets and can only host records for the IP addresses which belong to that subnet. In this case, if both blog.sortedit.net and mail.sorted.co.zw had IP addresses from the same subnet, e.g. 64.202.189.x then the pointer records for both domains can be created in the same Reverse Lookup Zone regardless of each servers domain membership.