Patent ID: 7940761

Claim:
An authentication method for use in a server computer for authenticating that a client computer connected to the server computer via a network is a legitimate client computer, if starting communication with the client computer, the method comprising: receiving, by the server computer, a plurality of connection request packets from the client computer; storing, in a storage of the server computer, a plurality of group identification information items for identifying, respectively, a plurality of groups, in relation to connection information indicating a number of connection request packets uniquely and secretly allocated to each of the groups, each group including a plurality of users allowed to access the server computer; counting connection request packets received from the client computer within a preset period to obtain a counted number; determining whether an information item included in the connection information corresponds to the counted number; authenticating a sender of the connection request packets if the information item included in the connection information corresponds to the counted number; generating a connection request acknowledgement packet in response to at least one of the connection request packets, if the sender is authenticated; and transmitting the generated connection request acknowledgement packet to the client computer.