If you can tie it in well, and show sincere interest, I would say that it could be helpful to include a second area.
In the computer science department I attended, one was expected to choose either two or three areas to specialize in. One was not enough! The department didn't want people graduating with too narrow a focus. Once you had sufficient background knowledge in an area, you took an exam in that area.
Look at the list of areas in the department you're applying to, and pick one that looks interesting. Perhaps Theory of Computing would go well with Computing Networks, for example. You can talk a little bit about the second area in your statement.
Note that you'll likely be busy with coursework and preparing for exams during at least your first year. Check in with the networking people from time to time, to renew your interest. As a professor gets to know you, (s)he may be more motivated to squeeze you in.