IPv6 Unspecified Address (::)

The IPv6 Unspecified Address: ::

The unspecified address `::' (all 128 bits set to zero) indicates the absence of an address. It is the IPv6 equivalent of IPv4’s 0.0.0.0.

Representations

Format	Value
Compressed	::
Expanded	0000:0000:0000:0000:0000:0000:0000:0000
CIDR	::/128

When Is :: Used?

Context	Meaning
Source address	Host has no address yet (e.g., during DHCPv6)
Listening socket	Bind to all interfaces (`::' = IPv6 equivalent of 0.0.0.0)
Routing table	Default route (::/0 = "all destinations")
Configuration	"No address configured"

Programming Usage

// Listen on all IPv6 (and often IPv4) interfaces
server.listen(3000, '::');

// Equivalent to IPv4:
server.listen(3000, '0.0.0.0');

Key Rules

1. Never used as a destination — you cannot send packets to ::
2. Valid as source only in specific situations (DAD, DHCPv6 solicit)
3. Not the same as ::1 — :: is unspecified, ::1 is loopback
4. Routers must never forward packets with :: as source

Duplicate Address Detection (DAD)

When an interface first comes up, it sends a Neighbor Solicitation from :: (unspecified) to verify its tentative address is not already in use:

Source: :: (unspecified)
Destination: ff02::1:ff[last 24 bits]  (solicited-node multicast)

If no reply is received, the address is considered unique and is assigned to the interface.