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

Solution 1:

:: 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

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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.

Solution 2:

:: 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.