:: also refers to "a whole heap" of zeros in an IPv6 address.

Due to the massive address space available with the IPv6 addressing scheme, its inevitable that many addresses will contain long sequences of zeros. In order to make writing these addresses easier, a special syntax is available to compress the zeros, the guidelines of which are:

  • The use of "::" indicates multiple groups of 16-bits of zeros.
  • The "::" can only appear once in an address.
  • The "::" can also be used to compress the leading and/or trailing zeros in an address.

For example the following addresses:

1080:0:0:0:8:800:200C:417A (a unicast address)
FF01:0:0:0:0:0:0:101 (a multicast address)
0:0:0:0:0:0:0:1 (the loopback address)
0:0:0:0:0:0:0:0 (the unspecified addresses)
may be represented as:
1080::8:800:200C:417A (a unicast address)
FF01::101 (a multicast address)
::1 (the loopback address)
:: (the unspecified addresses)

And remember kids, if you dont have access to it already, hassle your ISP as the end of IPv4 address space is nigh!

Rules and examples shamelessly copied from RFC2373 "IPv6 Addressing Architecture" http://www.ietf.org/rfc/rfc2373.txt