Editing Plan 9 from Bell Labs (section)

=== Combining the design concepts ===
Though interesting on their own, the design concepts of Plan 9 were supposed to be most useful when combined. For example, to implement a [[network address translation]] (NAT) server, a union directory can be created, overlaying the [[router (computing)|router]]'s {{mono|/net}} directory tree with its own {{mono|/net}}. Similarly, a [[virtual private network]] (VPN) can be implemented by overlaying in a union directory a {{mono|/net}} hierarchy from a remote [[gateway (telecommunications)|gateway]], using secured 9P over the public Internet. A union directory with the {{mono|/net}} hierarchy and filters can be used to [[sandbox (computer security)|sandbox]] an untrusted application or to implement a [[Firewall (computing)|firewall]].<ref name="unix-spirit-set-free" /> In the same manner, a distributed computing network can be composed with a union directory of {{mono|/proc}} hierarchies from remote hosts, which allows interacting with them as if they are local.

When used together, these features allow for assembling a complex distributed computing environment by reusing the existing hierarchical name system.<ref name="design-paper" />