When you want to visit a Web site, chances are you just type its name into your browser's address bar. You probably don't know the site's IP address. Luckily, you don't have to; the Domain Name System (DNS) takes care of this for you.

The Domain Name System is a distributed database that keeps track of domain names and their IP addresses. At one time, when there weren't so many hosts on the Internet, the database that translated names to IP addresses was maintained locally on each computer. Today, tens of thousands of DNS servers distributed throughout the Internet handle lookup requests to translate names to IP addresses.


Here's how it works: When you type a name, such as www.ab.com, into your browser's address bar, your computer sends a DNS lookup request to your local DNS server (typically hosted by your ISP). During its normal course of operation, the local server caches entries in resource records. So if there's a current cached entry for www.ab.com, the local DNS server returns the corresponding IP address (63.87.252.186) to your computer, and your browser connects to that address. If there's no current entry, the DNS server contacts a root server for the top-level domain (such as .com, .org, or .net). The root server contains a database with entries for the authoritative server for all domains.

In our example, the root server would report back to your ISP that the authoritative DNS server for www.ab.com is 198.6.1.18 (auth40.ns.uu.net). Your ISP's DNS would then contact that authoritative server, which would return our Web site's current address. The ISP passes the address to your computer, which in turn plugs it into your browser. Though a lot is happening, it all happens in the background, and usually in just a fraction of a second.

Things may not go so smoothly if you want to host an application that will be accessed remotely, such as a webcam. If you're a home user, there's a good chance your IP address is dynamic. That's not a problem if all you do is surf, download files, IM, and use e-mail. But if you want to be able to be found on the Internet, you need either a static address (which ISPs are reluctant to provide, or will do so only at an additional cost), or you need a way to update your domain's IP address dynamically.

To solve the problem, DNS was extended with new protocols so that host names could be resolved with changing IP addresses. The easiest way to take advantage of this technology, called Dynamic DNS, is through providers who will update your domain's DNS entry automatically each time your IP address changes. When you sign up for the service, you delegate DNS control for your domain to the DDNS provider, so that when someone attempts to connect, the provider is the authoritative DNS for your domain. To keep your DDNS provider up to date, a small program on your machine periodically sends an update packet that specifies your current IP address.

The major DDNS providers include DynDNS.org ( www.dyndns.org ), Tzolkin Corporation ( www.tzo.com ), and No-IP.com (www.no-ip.com). Pricing for DDNS services for custom domains (your own domain name) is the same at all three providers, at $24.95 per year. But DynDNS.org offers free service if you're willing to use a subdomain on one of their domains (such as "craigswebcam .dyndns.org"). Similarly, No-IP.com offers free service on one of their subdomains, supported by banner ads, or you can pay a one-time charge of $12.95 for a subdomain on one of their domains.

It's possible that your existing home router or webcam has built-in support for one of the major DDNS providers. Tzolkin's TZO, for example, is supported on a number of Linksys and Netgear routers. This is definitely worth checking into. Configuring your router or webcam to update your DDNS provider is preferable to running a program on your computer, since those appliances are more likely to be turned on all the time.