the depletion of the public IPv4 address pool may speed up the deployment of IPv6. The coexistence of the two versions of IP requires some transition mechanisms. One of them is 6to4 which provides a solution for the problem of an IPv6 capable device in an IPv4 only environment. From among the several 6to4 relay implementations, the following ones were selected for testing: sit under Linux, stf under FreeBSD and stf under NetBSD. Their stability and performance were investigated in a test network. The increasing measure of the load of the 6to4 relay implementations was set by incrementing the number of the client computers that provided the traffic. The packet loss and the response time of the 6to4 relay as well as the CPU utilization and the memory consumption of the computer running the tested 6to4 relay implementations were measured. The implementations were tested also under very heavy load conditions to see if they are safe to be used in production systems.