IPv6 addresses do have colon ":" in them, which makes it non-trivial to use it is URL syntax, since URLs also use colons as delimiters.

In case of MySQL-compatible URLs,
jdbc:mysql://<host>[:<port>]

having <host> as IPv6 would make challenging to correctly parse the URL.

At first, we'll follow standardized procedure described in http://www.ietf.org/rfc/rfc2732.txt, which solves the problem putting IPv6 address inside square brackets. For MySQL URLs it would look like e.g

jdbc:mysql://[::1]:3307/ (IPv6 localhost and port 3307)