VPN is a technology that realizes the security transmission of private information through the public network. As one of key technologies, tunnel technology resolves the mobility problem of mobile nodes. Firstly, L2TP and IPSec have been compared based on these two tunnel principles introduction of VPN. Secondly the advantages of IPSec in security have been introduced. Then, the problems of mobile host and two models of tunnel in VPN have been introduced. Lastly, the advantages of utilizing the mobile IP to resolve the problem of mobile host have been analyzed, and this technology has been compared with VPDN.