I am learning computer networks, but getting confused each time I proceed my analysis. Kindly help. Thanks in advance!

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    – Ron Maupin
    Dec 25 '18 at 8:31

That is the IPv6 unspecified address, and you use it when you do not know an address or have an address. The IPv6 unspecified address (::/128) is explained in RFC 4291, IP Version 6 Addressing Architecture, Section 2.5.2. The Unspecified Address:

2.5.2. The Unspecified Address

The address 0:0:0:0:0:0:0:0 is called the unspecified address. It must never be assigned to any node. It indicates the absence of an address. One example of its use is in the Source Address field of any IPv6 packets sent by an initializing host before it has learned its own address.

The unspecified address must not be used as the destination address of IPv6 packets or in IPv6 Routing headers. An IPv6 packet with a source address of unspecified must never be forwarded by an IPv6 router.


there are at least two uses of the "unspecified address" that I am aware of.

Firstly before assigning it's first (link local) ipv6 address a node is expected to perform duplicate address detection. To do that it must send a neighbour solicitation packet but it doesn't have any address yet.

Secondly the address is used as a wildcard in the "sockets" API for an application to tell the OS that it wants to listen on all addresses.

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