To host a web site at home, or host BitTorrent downloads at home, you need a way for others to find your computer on the Internet.

Right now, your computer at home -- or, more likely, your connection sharing device, such as a wireless router -- probably has a dynamic IP address. That means that other users can only refer to your computer by a cryptic address that doesn't mean much to human beings, such as 10.250.17.234. And if your IP address is dynamic, it changes every time you reconnect to your DSL or cable modem provider.

Even if your IP address is static, you still need a way for users to access your site by name, instead of by number... unless you're willing to put up with a URL like http://10.250.17.234/. And you might change ISPs. So I recommend following this article's recommendations anyway. If a URL like that doesn't bother you, and you're sure you won't change ISPs and that you have a static IP address, You may skip this step. If you don't know which you have, you probably have a dynamic IP address.

So how can we give out a reliable address to folks who want to connect to our web site or BitTorrent tracker... for free? Easily: we'll use a dynamic DNS service, such as DynDNS. DynDNS offers free dynamic and static DNS services.

I use DynDNS as an example here, but you should also consider similar services offered by www.no-ip.com.

Visit the DynDNS site, or that of a similar free DNS service, and set up a dynamic IP DNS account for yourself. DynDNS offers DNS subdomain names in several domains with rather entertaining names. For instance, you might pick exampleguy.is-a-geek.com.
What if you want to use your own domain name, instead of is-a-geek.com or another domain offered by DynDNS? In a nutshell: it's tougher, and it's usually not quite free, but you can do it! For the sake of readability, I've separated my answer to that question into a separate article. Check out How do I host a real domain name at home? for complete details.
You have a dynamic DNS account now... but so far, you're still stuck logging in to DynDNS every time your IP address changes! How can you fix that? By installing the DynDNS Updater software, which stays resident on your computer and automatically updates your IP address in the DNS records, so that yourname.is-a-geek.com follows you everywhere you go. Even if you change from cable modem to DSL or vice versa, your dynamic DNS will still be updated to point to the right place. Neat, huh?

You can get the DNS updating software from the DynDNS Update Clients page. Installation is a snap, with just two confusing pages in the setup wizard: "Hostname Group" and "Hostname Information."

If you have just one hostname, it's hard to see why either of these is useful. Fortunately, all you need to do on the "Hostname Group" screen is make up a name for your "group" of hostnames. exampleguy will do fine. And when you reach the "Hostname Information" screen, just be sure to check the box next to your one and only hostname before clicking "Next." Also be sure to enable automatic update -- that's the whole point!

In my tests the Dynamic Update software showed an IP address only valid on my local network in a little popup "tooltip" window -- but my "real" public IP address was set correctly on the dyndns server. So don't panic.

Got a Mac or a Linux system? You can still have automatic DynDNS updates! Try the "third party clients" link at the bottom of the DynDNS Update Clients page.

Now you have your very own hostname on the Internet... for free! For most people, the next step is to assign a static local IP address to your computer so that you have a fixed address on your home network to forward web or BitTorrent traffic to.