There is a design flaw with IPv6 when using URL with port. Because the : is used as part of the address, when we need to specify port number

We need to put [IPv6]:port

The extra bracket [] broke most of the existing application.

Because you are already using latest version of software that is IPv6 aware.

I said it broke *existing* software.

Example: If you are using http://192.168.0.1/index.html but changed to port 81 instead of the default 80.

What existing software will do is, http://192.168.0.1:81/index.html

But if you use IPv6, http://::1/index.html, becomes http://::1:81/index.html. It will not works. It has to be http://[::1]:81/index.html.

The requirement to add extra bracket [] broke almost all existing software. If IPv6 uses another characters instead of :, those old software that doesn't aware of IPv6 should still works. Because they can handle IPv6 just like IPv4.