main parts of URL

In general, URLs are written as follows:

     :

 A URL contains the name of the scheme being used () followed
 by a colon and then a string (the ) whose
 interpretation depends on the scheme.

 Scheme names consist of a sequence of characters. The lower case
 letters "a"--"z", digits, and the characters plus ("+"), period
 ("."), and hyphen ("-") are allowed. For resiliency, programs
 interpreting URLs should treat upper case letters as equivalent to
 lower case in scheme names (e.g., allow "HTTP" as well as "http").