A method of restricting access to data maintained on a server computer by one or more authorized, networked client machines includes the step of installing a client-side software program on the client machine for generating a client machine-specific identifier determined by particular characteristics of the client machine. The machine-specific identifier is used by the server administrator to generate a unique password, which the user enters into the client-side software program. Server-side software is embedded on protected Web pages of the server computer that hosts the protected Web site. When a user desires access to protected content, the client-side software is prompted to re-generate its machine-specific identifier and valid client password list for comparison with the password previously entered by the user. Access is granted if they correspond, and denied if they do not. If the client machine is recognized as being authorized to access data on the protected Web site during a first access request, then the current session identifier is saved in a temporary storage table remote from the client machine for indicating current working sessions of authorized client machines. The client machine returns such session identifier with each additional request for access, and the temporary storage table is consulted to search for such session identifier before granting access.