... a colon, if it's followed by a port number...
Minor nit: The colon and port number is just after the hostname in a URL, not the page name. For example, consider the port 8080 in
The question mark following the page name in a URL starts a list of parameters being passed from the browser to the script running in the server. The parameter values can be more or less anything; by convention spaces will have been replaced by plus signs, but otherwise almost anything goes, including colons. For example,