A network security device 10 is connected between a protected client 12 and a network 100. The network security device 10 negotiates a session key with any other protected client. Then, all communications between the two clients are encrypted. The inventive device is self-configuring and locks itself to the IP address of its client 12. Thus, the client 12 cannot change its IP address once set and therefore cannot emulate the IP address of another client. When a packet is transmitted from the protected host, the security device 10 translates the MAC address of the client to its own MAC address before transmitting the packet into the network. Packets addressed to the host, contain the MAC address of the security device. The security device 10 translates its MAC address to the client's 12 MAC address before transmitting the packet to the client 12.