NAT is a solution technique of IP address depletion problem. However, NAT does 
not maintains end-to-end connectivity because the NAT device modifies packet 
header information on the packet that passes oneself. Therefore, in the server-
client environment, when the server is inside the NAT device, the external 
client cannot communicate with the server directly.  It is necessary to 
configure a static port mapping to the NAT device to communicate with the server 
which is inside the NAT device. However, users in NAT do not have the privilege, 
users cannot configure static port mapping. Therefore, it is necessary for 
user to traverse NAT dynamically. This research focuses on DirectIP which is one 
of the functions of NAT-Traversal. And we propose improvement of DirectIP to use 
convenient for user.