Automatic tunnels use “IPv4-compatible” addresses, which are hybrid IPv4/IPv6 addresses. Adding leading zeros to the 32-bit IPv4 address to pad them out to 128 bits creates compatible addresses.
When traffic is forwarded with compatible addresses, the device at the tunnel entry point can automatically address encapsulated traffic by simply converting the IPv4-compatible 128-bit address to a 32-bit IPv4 address. On the other side of the tunnel, the IPv4 header is removed to reveal the original IPv6 address. Automatic tunneling allows IPv6 hosts to dynamically exploit IPv4 networks, but it does require the use of IPv4-compatible addresses, which do not bring the benefits of the128-bit address space.
When traffic is forwarded with compatible addresses, the device at the tunnel entry point can automatically address encapsulated traffic by simply converting the IPv4-compatible 128-bit address to a 32-bit IPv4 address. On the other side of the tunnel, the IPv4 header is removed to reveal the original IPv6 address. Automatic tunneling allows IPv6 hosts to dynamically exploit IPv4 networks, but it does require the use of IPv4-compatible addresses, which do not bring the benefits of the128-bit address space.
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