Microsoft® Manual of Style, Fourth Edition

URLs, addresses

A uniform (not universal) resource locator (URL) is an address, specified in a standard format, that locates a specific resource on the Internet or an intranet. In content for a general audience, use address rather than URL. In content for a technical audience, do not spell out URL on first mention.

The appropriate indefinite article for URL is a, not an.

A URL consists of an Internet protocol name; a domain name; and optionally other elements such as a port, directory, and file name. Use lowercase for each of these main elements, unless case is important.

In a typical URL, separate the protocol name (such as http:) from the rest of the destination with two slash marks, and separate the domain name and other main elements from each ...