Unspecified Address - ::/128

An IPv6 address with all 128 bits set to zero is called the unspedified adddress (which correspond to in IPv4).  This address should not be assigned to any host and it should only be used as the source address by initializing host before it has learned of its own address.  If a host listens for incoming connection on this address, it means that the host and/or its application is listening on all interfaces belong to that host.

In terms of networking, this ::/128 address indicates the default route much like that with subnet mask of in IPv4.  When a packet's destination doesn't match with any route int the routing table, the packet will be sent out using the interface associated with this particular address.

All 128 bit of the of an unspecified address is set to all 0's resulting an IP address that takes the form of 0:0:0:0:0:0:0:0:/128 or ::/128 for short.