Now you should have a rough picture of the complexity involved in managing a domain name -- there are at least 3 sets of computers involved!

• One set belongs to your domain name registrar. They keep track of what domains are registered, who owns them, and what computers manage the DNS settings for each of these domains. (This information is stored in a big database called the "whois" database).
• One set belongs to the company that manages the DNS settings for your domain. These computers understand what computer addresses correspond to what domain names. Other computers, like your web browser, can ask them to look up the name for an address, or vice versa.
• The third set belongs to your web and email hosting company. On these computers, your web site files are stored and your email is delivered. There are almost always different computers than the ones that handle the DNS lookups.

An "MX Record" is a DNS entry that indicates what server(s) handle the email messages for your domain. These can be, and usually are, different servers than those than handle your web site. "MX" stands for "Mail Exchange". Typically, you will have 2 or more MX records for your domain. One is primary; the others are secondary and will queue your email temporarily if your primary email server is down, forwarding it to your primary server when it comes back up. We strongly recommend using 2 or more different MX records.

If John Sample registered "sample.com", then he really can have any number of domain names, as long as they each end in "sample.com". I.e. "john.sample.com", "x1234.sample.com", and "www.xqy.apple.sample.com" are all domains that John has a right to setup and use because he has registered "sample.com". These are all called "subdomains" because you cannot register them individually, but get them if you register the normal domain "sample.com". Subdomains are created when entries for them are made in the DNS settings for your domain.

You can configure your DNS settings to use any addresses you wish for web and email for any of your subdomains. Your DNS provider should allow you to do this as a matter of course. However, your web or email hosting provider will probably charge you extra for the use of additional web sites (for each subdomain) or additional email domains.

If your subdomain is configured to point to another domain name or subdomain name, rather than to a computer's address, it is known as an "alias" or a "CNAME".

The "Time-To-Live" or TTL is an important DNS setting that you should be aware of when you want to change your DNS name to address settings or to change the company that manages your DNS. A TTL is roughly the time, measured in minutes, hours, or days, that it can take for any change in your DNS settings to take effect all throughout the Internet. A small setting such as 20 minutes will cause all your settings to propagate across the Internet in about 20 minutes or so, a large setting can result in the changes taking days to be noticed. A typical default setting can be 6 to 24 hours! Clients for whom LuxSci manages their DNS generally have their TTLs set to 3 hours, unless they request otherwise. Note that the TTL is also the time it will take for changes in the TTL to take place...!

Why are your changes not instantly available? The answer reflects the clever way in which DNS works. Your changes ARE available instantly on the actual computers that manage your DNS. However, they aren't the only computers that give out DNS information to the world -- there are hundreds of thousands or even millions of these DNS servers around the world. How would an end user know to ask your particular DNS server for your particular address? That would require another type of "phone book"! No. Instead, an end user just asks their nearest DNS server for your address. This DNS server is probably owned by their Internet Service Provider. If this DNS server knows your DNS information, it tells the end user. If it doesn't know, it effectively asks your server, tells the end user and stores a copy of the information in case someone asks again.

This distributed method of looking up DNS information is good because it is fast and minimizes the work your DNS servers have to do. It has the drawback that the other DNS severs have stale information whenever you change the data on your servers. To compensate, you can set the TTL on your DNS settings. Effectively, if a DNS server has information that is older than its TTL, the DNS server doesn't trust that the data is accurate and goes to get a fresh copy. This is why the time it can take your DNS changes to propagate across the Internet is approximately the TTL settings you have made for your domain.