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What is DNS? How does it work?

The Domain Name System (DNS) is the way that Internet domain names are located and translated into IP addresses. As you know, the internet is actually routed by computers, not people, and computers are good with numbers. Every computer on the Internet can be reached by routing to a unique number which designates its domain and host. Humans, on the other hand, are lousy with numbers, but a little better with names. A domain name is a meaningful and easy-to-remember "handle" for an Internet address. It is far easier to remember "ftp.netscape.com" and "www.yahoo.com" than it is to remember and DNS is the system of taking the canonical host name (the whatever.com or whoever.org) and matching it up with an IP address which TCP/IP can use to route packets.

DNS has another advantage, which is secondary to its initial intent of making it easier to remember Internet addresses. With DNS in place, users can always point to "www.yahoo.com", but the DNS entries can change to specify different IP addresses. So when one machine goes out of rotation and another comes in, the user will not notice a change at all. They are still pointed at "www.yahoo.com" though the actual box and IP address have changed. Furthermore, DNS entries can be set up so that several names point to the same IP or one name can point to one of several IPs in a "pool". If you do a lookup on www.yahoo.com, you should see multiple IP entries. This allows the load of hits to that site to be spread out over several different machines. If one of these machines fails, the others can pick up the slack in the background.

Maintaining one big central list of domain name/IP address correspondences would be impractical and never up to date. Instead, the lists of domain names and IP addresses are distributed throughout the Internet in a hierarchy of authority. Each domain has its own authoritative DNS server (the one it actually makes changes to) and that server shares its information with other DNS servers on the Internet. This is done primarily with a program called BIND that takes a slice of the server's DNS record and exchanges it for a slice of another DNS server's record.

When a user experiences a "DNS issue", the symptom is always the same: they can't find www.yahoo.com, but they can put into a browser and it comes up fine. If this is a problem with all the sites they try, they are not set up to use the correct DNS servers, or they cannot communicate correctly with the DNS servers. If this only happens with one or a handful of sites, the problem may be with caching on their machine (Eudora is bad about this, or the "hosts" and "lmhosts" need to be wiped from the computer), or it is a problem with an out-of-date DNS entry. Sometimes the problem is with the ISP's DNS records. Many times changing access numbers will fix this, but only if the ISP uses multiple network providers (like EarthLink). If this fixes the problem, the users normally needs to report the problem to their ISP so the problem can be fixed.


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