A closed source proprietary software can communicate with a GPL'd software via inter-process communication, including network communication over TCP/IP, without being subject to the terms of the GPL, if it's not exchanging intimate internal data structures that is found in the source code of the GPL'd software.
In this case, the closed source proprietary software is a mere aggregate and not a derivative of the GPL'd software.
From the GPL FAQ:
What is the difference between an “aggregate” and other kinds of
“modified versions”?
An “aggregate” consists of a number of separate programs, distributed
together on the same CD-ROM or other media. The GPL permits you to
create and distribute an aggregate, even when the licenses of the
other software are nonfree or GPL-incompatible. The only condition is
that you cannot release the aggregate under a license that prohibits
users from exercising rights that each program's individual license
would grant them.
Where's the line between two separate programs, and one program with
two parts? This is a legal question, which ultimately judges will
decide. We believe that a proper criterion depends both on the
mechanism of communication (exec, pipes, rpc, function calls within a
shared address space, etc.) and the semantics of the communication
(what kinds of information are interchanged).
If the modules are included in the same executable file, they are
definitely combined in one program. If modules are designed to run
linked together in a shared address space, that almost surely means
combining them into one program.
By contrast, pipes, sockets and command-line arguments are
communication mechanisms normally used between two separate programs.
So when they are used for communication, the modules normally are
separate programs. But if the semantics of the communication are
intimate enough, exchanging complex internal data structures, that too
could be a basis to consider the two parts as combined into a larger
program.
