Because Worldwide Web documents are designed to distributed around the World, a standard naming scheme is required. This is the Uniform resource Locator (URL) and it describes four things so that the client can go to the right server and bring back the correct file.
These are wrapped up in a standard form: protocol://computer:port/filename
Thus the full URL for the Oxford University Network Information Service can be written as http://www.ox.ac.uk:80/index.html
In fact, because most of the Internet service servers do run on their standard port, it is only necessary to use the port number in the URL if explicitly given. Another default is that many HTTP servers use a default filename of index.html if none is given which leaves http://www.ox.ac.uk/. And even the often, the trailing slash is not necessary. Giving http://www.ox.ac.uk.