IPv6 rapid deployment (6rd) builds upon mechanisms of 6to4 (RFC3056) to enable a service provider to rapidly deploy IPv6 unicast service to its existing IPv4 sites. Like 6to4, it utilizes stateless IPv6 in IPv4 encapsulation in order to transit IPv4-only network infrastructure. Unlike 6to4, 6rd requires a service provider to use one of its own IP prefixes rather than the fixed 6to4 prefix. A service provider has used this mechanism for its own "rapid deployment" of IPv6 (five weeks from first exposure to "opt-in" deployment for 1,500,000 residential sites).