1. A method for selectively modifying packets or the transmission of packets transmitted between a first layer in the ISO/OSI model in a network system and a second layer in the ISO/OSI model in a network system, the first layer in the ISO/OSI model being different from and adjacent to the second layer in the ISO/OSI model, the network system including a first process for translating packets from said first layer in the ISO/OSI model to said second layer in the ISO/OSI model, the packet having a plurality of properties including a packet size, a packet offset and a plurality of packet traits, the method comprising the steps of: A) receiving from a user a set of user-specified conditions and a user-specified operation associated with a set of user-specified conditions by: A1) generating a first display of a first plurality of user interface controls for specifying said set of user-specified conditions; A2) receiving through user interaction with said first plurality of user interface controls user input that specifies said set of user-specified conditions, wherein each of said user-specified conditions designates an arithmetic relationship between a property of packets and a specified value; A3) generating a second display of a second plurality of user interface controls for specifying said user-specified operation associated with said set of user-specified conditions; A4) receiving through user interaction with said second plurality of user interface controls user input that specifies said user-specified operation, said user-specified operation being an operation that either modifies the packet or modifies transmission of the packet; B) causing a second process to execute when an attempt is made to transmit a packet from said first layer in the ISO/OSI model to said second layer in the ISO/OSI model, said second process performing the steps of: B1) inspecting said packet to determine whether the packet satisfies said set of user-specified conditions; B2) if said packet satisfies said set of user-specified conditions, then performing a user-specified operation on said packet; and C) invoking said first process after executing said second process.