This address is called an FQDN , or Fully Qualified Domain Name . A computer's FQDN can be used from anywhere on the Internet to identify the computer and translate it into an IP address. Note that FQDNs are case-insensitive. Most of the time, they are written in lowercase, but this doesn't mean they have to be. The third part of the URL, which is optional, is the port number .
Internet hosts have a certain number of ports.
You can think of them as those booths you see in a bank. It's the same with Internet hosts - one port could offer HTTP, another could offer mail routing, and so on. Ports are numbered, and most services have a pre-defined port that they usually work from. HTTP usually runs on port 80, but this is not necessary. If it is running on port 80 in the machine you named in the hostname part, then you don't need to specify a port number.