What is the maximum number of colons ':' that may appear in a valid URL?

In idle conversation I was asked by MM.

Question: What is the maximum number of colons ‘:’ that may appear in a valid URL?

* If you said zero to one, then you are victim of browsers, and you have never used anything but a browser.

* If you said one, then your a novice.

* If you said two, then you have probably seen http://host:port at some time.

* If you said three, then you would be correct, the elite.

http://user:pass@host:port/location

For the record my initial answer was 2.