Transition Mechanisms - 6-to-4 Tunnelling

6to4 tunnelling allows v6 packets to travel across v4 clouds by using 6to4 gateway routers to configure a special address and prefix.  The address prefix 2002: is reserved for 6to4 based addresses.  Added to this address is the v4 address which is 32 bits making an end prefix of /48, the next 16 bits are used for v6 subnets which allows 65,536 subnets

6to4 relay routers are used to connect 6to4 networks to native v6 networks with minimal configuration.  These routers use an anycast prefix address of 192.88.99.0/24.  6to4 packets can be sent to any neighbouring router as long as the router supports 6to4 tunnelling.  This also means that if a 6to4 router was to fail between subnets then the traffic would simply be sent to the next 6to4 router with the same address prefix, no manual re-configuration is needed.  Traffic from a v4 network is given the anycast address of 192.88.99.x.  Traffic from a v6 network sending to a 6to4 cloud is given the v6 equivalent of the 6to4 anycast address of 2002::/16.  Since 6to4 addresses only require the use of a v4 address it is likely to be used in the transition period of the co-existing v4 and the inevitable v6 infrastructure.  6to4 tunnelling does not require IPv6 gateway routing protocols as the IPv4 protocols perform these tasks.

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