6to4: A simple Overview.

“6to4 is an Internet transition mechanism for migrating from IPv4 to IPv6, a system that allows IPv6 packets to be transmitted over an IPv4 network (generally the IPv4 internet) without the need to configure explicit tunnels.

Special relay servers are also in place that allow 6to4 networks to communicate with native IPv6 networks.

6to4 is especially relevant during the initial phases of deployment to full, native IPv6 connectivity, since IPv6 is not required on nodes between the host and the destination. However, it is intended only as transition mechanism and is not meant to be used permanently.” (Wikipedia)

This means that there are a number of “relay servers” (6to4 uses the address 192.88.99.1, or in IPv6 notation 2002:c058:6301::) available advertised as anycast addresses.
In other words, the address is available in many places across the internet however everyone will be simply be “routed” to the closest one.

6to4 was created to give to any public IPv4 address a matching IPv6 allocation.

It essentially performs the following functions:
– Allocates a /48 block of IPv6 space to any public routeable IPv4 address
– Encapsulates IPv6 traffic inside IPv4 packets to allow it to be transmitted over the IPv4 network to the nearest 6to4 relay.
– Routes traffic between 6to4 and native IPv6 networks

The prefix 2002::/16 has been allocated specifically for 6to4 hosts, to work out an IPv6 space simply:

1. Take public IPv4 address.
2. Convert the address from dot-decimal notation to HEX notation.
3. Insert this as the section after the 2002: prefix, making sure to split it into the correct grouping (each group represents 16 bits or 2 octets).
4. Complete the address as a /48 block (2002:XXXX:XXXX::/48)

This is the IPv6 allocation, which has 65536 /64 subnets it can be divided into.

To use the obtained address:
1. Setup a 6to4 interface.
2. Add a default route to the global IPv6 Internet through the tunnel interface using the anycast IPv4 address.
3. Add IPv6 addresses as required in the LAN Segment.


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