How does the sub-domain like urls are created in web apps?

I registered in a site called basecamp. After I registered, they
gave me a url like where i have to log in access
my project management. I heard that basecamp is built in ruby on rails.
I couldn’t get the point of building a subdomain like url. Can anyone
give some guidance of how to create a sub domain like url using rails?
Thanks in advance

You can use subdomains for various purposes. One is to host different
servers (eg and, or to host
different applications (eg [email protected] and [email protected]).

I believe Basecamp use it to have nicer URLs. The standard way would
//…, or

//… looks nicer.

A few links: