[kazinator]

The IPv6 notation claims : for separating digits. But that's already an established notation for port numbers: 10.1.2.3:456. Oops!

The square brackets allow us to stick a port number on it: [ffff::0123:4567]:6301

[jeffbee]

Yes, but the RFCs permit 6v4 addresses such as ff:ee::aa:1.2.3.4