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We present many examples of client/server applications in this manual, most of which assume that the client and server programs are running either on the same host, or on multiple hosts with no network restrictions. We can justify this assumption because this is an instructional text, but a real-world network environment is usually much more complicated: client and server hosts with access to public networks often reside behind protective router-firewalls that not only restrict incoming connections, but also allow the protected networks to run in a private address space using Network Address Translation (NAT). These features, which are practically mandatory in today's hostile network environments, also disrupt the ideal world in which our examples are running.