Hello Balijt,
an ipv6 address is 128 bit in size.
It is expressed as groups of 4 hexadecimal digits.
four hex digits at 0 can be compressed as 0
:0000: :0:
leading zeros can be omitted
:0bcd: can be written as :bcd:
to be able to indicate an ipv6 address without ambiguity the :: can be used only once so by looking at the other groups of hex digits is possible to understand how the :: pattern has to be expanded.
::1ab = 0000:0000:0000:0000:0000:0000:0000:01ab
Hope to help
Giuseppe