Recently, along with the proliferatin of mobile devices connected through Internet, a secure and practical mobile IP implementation became an important issue. A mobile IP can not work properly without authentication between the mobile node (MN), the home agent (HA) and the foreign agent (FA). In this paper, we propose a practical mobile IP authentication protocol with public key cryptography only during the initial authentication. The proposed scheme is compatible with the conventional mobile IP protocol and provides scalability for the number of MN's. We also showed the secure operation of the proposed protocol and performance evaluation of the proposed protocol was made.