Abstract: Virtual private network is the key technology of next generation Internet, L2TP is a dialup virtual private network specification which was defined by IETF. Its design and implementation are very significant. From the L2TP, basic technology and two kinds of servers of L2TP(LAC server and L2TP net server) are introduced and L2TP function models(authentication model、log model and LAC model) on this are expatiated. Based on this, a L2TP scheme was suggested and developed on a few UNIX latforms.