The Mach operating system supports secure local communication within one node of a distributed system by providing protected communication abstraction can be extended over a network by Network Server tasks. The network servers effectively act as local representatives for remote tasks by implementing an abstraction of Network Ports. To extend the security of the port-based communication abstraction into the network environment, the network servers must protect both the messages sent over the network to network ports and the access rights to network ports. This paper describes in detail the protocols used by the network servers to support security.