Curiously, this appears to be a bug in Windows Phone. In URIs, part following `//` is called authority, which is essentially a host with some optional additional stuff (like port number).
According to RFC 1123, hostname could legally be entirely numeric, and web browser shouldn't attempt to "correct" it (as it is a valid URI) for schemas it doesn't know anything about - as it doesn't know the rules for hostname for a given protocol. This is also not a valid IP address according to RFC 3986 (which specifies URI syntax), as this specification requires #.#.#.# format with three dots.
That said, using authority for something that isn't technically a hostname is misusing the field. I think using `<scheme>:<integer>` would have been a better idea.
According to RFC 1123, hostname could legally be entirely numeric, and web browser shouldn't attempt to "correct" it (as it is a valid URI) for schemas it doesn't know anything about - as it doesn't know the rules for hostname for a given protocol. This is also not a valid IP address according to RFC 3986 (which specifies URI syntax), as this specification requires #.#.#.# format with three dots.
That said, using authority for something that isn't technically a hostname is misusing the field. I think using `<scheme>:<integer>` would have been a better idea.