There is provided a route optimization method in a mobile Internet Protocol (IP) system using a mobile Internet Protocol. When a mobile host (MH) moves over the Internet and is assigned to a care-of-address (CoA) corresponding to a changed point of attachment to the Internet, it sends a registration request message containing the care-of address to a home agent (HA) via a foreign agent (FA). The home agent stores the care-of address in relation to a domain name of the mobile host in an internal domain name server database. Upon receipt of a query about the mobile host by domain name from a correspondent node (CN), the home agent sends the care-of address corresponding to the domain name of the mobile host together with caching prevention information to the correspondent node. The caching prevention information is set to prevent the correspondent node or another domain name server from caching the care-of address of the mobile host. The correspondent node communicates with the mobile host using the care-of address.