A multi-level network security system is disclosed for a computer host device coupled to at least one computer network. The system including a secure network interface Unit (SNIU) contained within a communications stack of the computer device that operates at a user layer communications protocol. The SNIU communicates with other like SNIU devices on the network by establishing an association, thereby creating a global security perimeter for end-to-end communications and wherein the network may be individually secure or non-secure without compromising security of communications within the global security perimeter. The SNIU includes a host/network interface for receiving messages sent between the computer device and network. The interface operative to convert the received messages to and from a format utilized by the network. A message parser for determining whether the association already exists with another SNIU device. A session manager coupled to said network interface for identifying and verifying the computer device requesting access to said network. The session manager also for transmitting messages received from the computer device when the message parser determines the association already exists. An association manager coupled to the host/network interface for establishing an association with other like SNIU devices when the message parser determines the association does not exist.