IPv4-Mapped IPv6 Addresses (::ffff:0:0/96)

IPv4-Mapped IPv6 Addresses

IPv4-mapped IPv6 addresses embed an IPv4 address within the IPv6 address format using the prefix ::ffff:. They are used by dual-stack systems to represent IPv4 addresses in an IPv6 context.

Format

::ffff:a.b.c.d

Examples:
::ffff:192.168.1.1    (192.168.1.1 in IPv6 format)
::ffff:10.0.0.1       (10.0.0.1 in IPv6 format)
::ffff:127.0.0.1      (loopback in IPv6 format)

Expanded Form

::ffff:192.168.1.1

Expanded:
0000:0000:0000:0000:0000:ffff:c0a8:0101

Where c0a8:0101 = 192.168.1.1 in hex:
  192 = 0xc0, 168 = 0xa8, 1 = 0x01, 1 = 0x01

Binary Representation

80 bits of zeros | 16 bits of ones | 32-bit IPv4 address

0000...0000 : ffff : c0a8:0101
(80 zeros)   (16 ones) (IPv4 in hex)

How Dual-Stack Uses Them

When an application creates an IPv6 socket:

1. Client connects via IPv4 to server
2. OS maps incoming IPv4 address to ::ffff:x.x.x.x
3. Application receives the IPv4-mapped address
4. Application can handle both IPv4 and IPv6 through one socket

Programming Example

// Node.js server on IPv6 socket receives IPv4 connection:
server.on('connection', (socket) => {
  console.log(socket.remoteAddress);
  // "::ffff:192.168.1.100" for IPv4 clients
  // "2001:db8::1" for native IPv6 clients
});

Important Notes

• IPv4-mapped addresses are not sent on the wire — they are an internal OS representation
• The actual network traffic uses standard IPv4 packets
• Some applications strip the ::ffff: prefix when displaying addresses