DNS 101

DNS or the domain name service is a piece of software which uses a lookup table to relate computers on a physical network to domain and computer names. DNS sits on top of TCPIP or Transfer Control Protocol/Internet Protocol where each individual NIC (network interface card) in a computer in a network is given a unique 4 number code a typical code may be 192.68.131.202. For more information please see my article ‘Networking 101’.For example I can set up a LAN and have all the computers in it in a domain called simonspace.net. Now let us say that I give one of the computer on the domain the name mycomputer then the full name for this computer would be mycomputer.simonspace.net.I would set up one machine to be the DNS server which contains a lookup table with a list of the names of computers in the domain along with their TCPIP addresses for example mycomputer may be listed next to the 192.68.131.202. This means that if I ping the DNS server with the name mycomputer I will get back 192.68.131.202. It is also possible to give the IP address and get back the name of the computer with this address, this is known as a reverse lookup.In reality on a single domain I do not have to bother about the extension .simonspace.net because all machines are on the same domain. However on an intranet with multiple domains the situation would be different. Let’s say you have a company ‘ACME Products’ with a production office in one area and a sales office in another.In the sales office there is a LAN with a domain called sales.acmeproducts.com in the production office there is another LAN with a domain called .production.achmeproducts.com. The two LANs are connected by two internet routers over the internet using VPN tunneling so that the networks are not available to the public. A network consisting of two or more LANs connected together via gateways is called an intranet.If there is a computer on the production network called mycomputer and I am sitting on a computer on the sales network I can still access the address of mycomputer on the other network but this time I have to ping the other mycomputer using its full intranet domain path, .mycomputer.production.acmeproducts.com. How does this happen?The computer I am at queries its domain controller with the name mycomputer.production.achmeproducts.com, this computer is not in its domain so it looks in its cash of known DNS servers and finds .server.production.achmeproducts.com. This server looks in its own domain and finds .mycomputer.production.acmeproducts.com and returns the corresponding IP address.Now you might say that it is not necessary to have the acmeproducts.com extension, why not just let the .sales domain and the .production domain communicate with each other? This would be true if all communication was always private through the VPN tunnel but what if I want a public presence?Let’s say I am the sales manager for acme products and I have an account on the sales.acmeproducts.com domain. I want customers to be able to e-mail me when they are looking at the contact us link on the acme products web site. So a web server is created outside the company domain firewalls with limited access to the sales and production domains.The customer can now go to the website and click on the link to my e-mailmanager@sales.acmeproducts.com and send me an e-mail. Now lets say I just gave you my business card with my e-mail address on it and you wanted to send me an e-mail. Your local internet service provider (ISP) knows nothing about acme products but it knows all about DNS servers for .com. Does this mean that there is a .com domain? The answer is yes.When the achmeproducts.com domain was set up it must have been registered with the .com domain, which would then put it on the list of sub domains in the .com domain. So what happens to the e-mail that you want to get to me?The request is first given to a DNS server for the .com domain this in turn is passed on to a server for the .acmeproducts.com domain which finally passes it to the server for the .sales.achmeproducts.com domain which contains the manager@sales.achmeproducts.com account.So there you have it DNS is the mechanism that simplifies communication in all TCPIP networks, up to and including the internet itself.Wishing you best of luck with all your IT projects, Simon.

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