Abstract: Many attacks may be carried out against communications in Local Area Networks (LANs). However, these attacks can be prevented, or detected, by providing confidentiality, authentication and data integrity services to the exchanged data. This study introduces a transparent security system fOr LAN communications (TSSOLAC) that protects the communications in a LAN of Microsoft Windows machines from possible security attacks. On each host in the protected LAN, TSSOLAC transparently intercepts each outbound IP (Internet Protocol) packet and inserts a crypto header between the packet IP header and payload. This header is used to detect any modification to the content of the packet in transit and to detect replayed packets. Then, the system encrypts the IP packet payload and some fields of the inserted crypto header. On the other hand, the system transparently intercepts each inbound IP packet, decrypts its encrypted portions and then uses its crypto header to authenticate the packet. If the packet is properly authenticated, the system indicates it to upper protocols. To be transparent to applications, the TSSOLAC part that processes inbound and outbound IP packets is implemented as a NDIS (Network Driver Interface Specification) intermediate driver that resides between the Logical Link Control (LLC) and Medium Access Control (MAC) data link sublayers. The study presents the design, implementation and operation of TSSOLAC.