If you have a web/email server connected to the Internet that you want other to reach, that server must have a public IP address. Most Internet users don't even try to remember the IP addresses of sites they use, instead they use the Universal Resource Locator system of typing www.cellstream.com or something like this into their web browser or mail client. These Internet 'names' are translated by Internet appliances by requesting the IP address associated with the Internet "name" - very similar to looking up some one's name in a phone book and getting their phone number. The look-up is done with something called a Domain Name System (DNS for short).
To be listed in the DNS of the Internet, you must have one of those public IP addresses. These cost money, and are leased from your service provider. Soemtimes they can be very expensive. I have see some service providers charge upwards of $80/mo. for a "static", or more appropriately "fixed public" IP address.
One great trick to this is to not pay that monthly charge, and use a Dynamic DNS service. [
There are some limits/differences you should be aware of. First, when you get one of these services, you generally have to have their Internet name as part of your Internet name as you are now a child under their infrastructure. For instance, if you used TZO, your URL would be something like cellstream.tzo.net instead of cellstream.com. For marketing purposes, this may or may not be good. So some of the services I listed offer more expensive custom URLs.
So let's say you elect to go this route. You register on the dynamic dns site, pay your fee, then download and install the client. There is one final and critical step. You must now configure your router/gateway (Linksys, Netgear, D-Link, etc.) to properly forward the public traffic to your server(s). To do this, log into the Administrative Control panel of your device. Then look for the DDNS screen or settings. You can now represent your entire network via the dynamic DNS service. A second option is to leave this disabled, then look inside the Applications and Gaming area where you can port forward certain ports to one of your private IP addresses. You can make an entry here, for instance, to forward all Well Known ports (1-1024) to the private IP (192.168.X.Y) of your server. Either way, your web server(s) will now respond to the URL from your dynamic DNS provider.