What does [::] mean as an ip address? Bracket colon colon bracket

:: can be used once in an IPv6 address to replace a consecutive blocks of zeroes. It can be any length of zeroes as long as it is greater than a single block. All zeroes in a single block can be represented by :0: instead of writing out all four zeroes.

In this case, it means all zeroes, or the IPv6 equivalent of the IPv4 0.0.0.0

As an example of something that is not all zeroes:

fe80:0000:0000:0000:34cb:9850:4868:9d2c

Which is properly "reduced" to:

fe80::34cb:9850:4868:9d2c

As an example, it can also be written as:

fe80:0:0:0:34cb:9850:4868:9d2c

but that is far less common than just "double coloning" it.

:: is the IPv6 "unspecified" address, equivalent to 0.0.0.0 - the [] is notation to separate the address from the port specifier.

A program bound to :: will be given traffic for any actual IPv6 address assigned to the system - it may also receive IPv4 traffic too in the form of IPv6-mapped IPv4 addresses (::ffff:x.x.x.x) although this is dependent on the socket options set by the application.