>>22 Plan 9 does not have system calls for the multitude of communication protocols or device driver interfaces. For example /net is the API for all TCP/IP, and it can be used even with scripts or shell tools, writing data to control files to write and read connections. Relevant sub-directories like /net/tcp and /net/udp are used to interface to respective protocols. You can implement a NAT by mounting a /net from a perimeter machine with a public IP, while connecting to it from an internal network of private IP addresses, using the Plan 9 protocol 9P in the internal network. Or you can implement a VPN by mounting a /net directory from a remote gateway, using secured 9P over the public Internet.
Here would be an example of using union (a stack) directories in /net: just like inheritance in OOP, you can take one (possibly remote) /special directory and bind another local special directory on top of that, adding some new control files and hiding others. The union directory now is like a child object instance of the original parent. The functionality of the original can be partially modified. Consider the /net file system. If you modify or hide its /net/udp sub-directory you may control or extend the UDP interface with local filter processes, still leaving the original /net/tcp running intact, perhaps in a remote machine. Note that name space is per process: if you give an untrusted application a limited, modified /net union directory, you restrict its access to the net.