Let’s try the longest example possible first. Let’s say that I am sitting at my client computer, running Internet Explorer, and I want to view www.2000trainers.com. My client cannot contact this server until the name is resolved to an IP address. As much, my client queries my local DNS server (whichever DNS server is specified in the TCP/IP properties) and asks for the IP address associated with www.2000trainers.com. Since my local DNS server is not responsible (authoritative) for the 2000trainers.com domain, it passes the query to a root server. The root server gets the query, but only processes it partially. It sends my local DNS server back an answer on where to find a name server that know all about things that end in .com. My local name server caches this information, and then queries the .com name server, asking for the IP address associated with www.2000trainers.com. Again, the .com name server gives only a partial answer, sending back the information on where to find the name server that knows all about things that end in 2000trainers.com. My local name server then caches this information, and queries the 2000trainers.com name server, looking for the IP address associated with a host called www. The name server looks up this record (since it is authoritative for things that end in 2000trainer.com) and returns the IP address 176.23.4.5 to my local DNS server. This information is cached, and then passed to the client (who also caches it), and the client can now communicate with www.2000trainers.com directly.
How long are those records that were cached stored for? I don’t know. However long the name server who gave me the answer says they can be stored for. Who knows better than the name server that is resposible for 2000trainers.com how often the name to IP address mappings change? Usually the records are cached for around a day, but sometimes less, especially if changes happen frequently. As such, if someone else were to query my local name server 3 hours later looking for www.2000trainers.com, the answer would be provided immediately from cache. By the same token, if my mail server were looking for the mail.2000trainers.com server, it would simply query my local DNS server, who would query the 2000trainers.com DNS server (since it has recently cached where this is located), who could provide the information relating to a host called mail. Note that another type of DNS server exists that is not responsible for any zone. These are called caching-only servers – they simply forward queries to other name servers and cache answers as outlined above, but are not authoritative for any zone. DNS is actually quite simple and straighforward. Don’t let the fact that you may never have used it before bother you. If you understand what I’ve outlined above, you understand how it works. We’ll worry about configuring it later.
DNS is implemented as a service in Windows 2000 Server, and as such can be started or stopped like any other service. It can also be added or removed using the Add/Remove Programs Windows Components Wizard. DNS is not installed automatically when you install Windows 2000, so it needs to be added manually. The number of DNS servers present on your network will depend a number of factors including your needs for fault tolerance, performance, and so forth. DNS is required in order to install Active Directory, since Active Directory domains follow DNS naming conventions. Note that the previous example was talking about DNS resolution out on the Internet. In the same manner, DNS can be used strictly for internal hosts, or a combination of both, so keep this is mind.
