Strictly speaking, a URL begins with a scheme followed by a colon. Schemes can be registered with IANA (or not), and everyone knows the most common half-dozen

Strictly speaking, a URL begins with a scheme followed by a colon. Schemes can be registered with IANA (or not), and everyone knows the most common half-dozen or so. People often forget "mailto:" and "tel:". The project brief asks for one thing, but the practical implementation probably requires something else. This is a good lesson to learn, and this is how two hours becomes two days, becomes two weeks.