Is there any Stack Exchange site where I can ask questions about computer networking project ideas?
Your question is vague, but I'll try and point you in the right direction.
If you mean network programming, I would look at Stackoverflow.
If your looking into networking because of a game (like minecraft networking or something like that...) check out GameDevelopment.stackechange.
You asked for a website that didn't need to be from stackexchange, so here is an amazing tutorial on networking.
There generally aren't any Stack Exchange communities which serve to help people develop their ideas. (Though I could be wrong, there are more and more sites being created these days.)
Stack Exchange's question and answer format is geared more toward the implementation, not the idea. There are certainly a number of less concrete questions on Stack Overflow, and even more on Programmers, but the focus remains pretty clear. These sites are about solving discrete problems, not discussing ideas.
For example, if your question was:
Then your question would be closed. It doesn't belong on any Stack Exchange site (that I know of).
However, if your questions was:
Then your question would be a better fit.
Notice the main difference is in the amount of work you put into the question. Open-ended discussions don't make for good questions on Stack Exchange. But discrete reproducible problems with specific parameters make for great questions.
A few things that should go into any given question in most cases:
You can see where this is going. The more details you present, the more help can be provided. It's especially helpful to give a complete example to reproduce the issue. For example, with code, don't just post all of your code. Remove anything unrelated to the problem. Reproduce the problem with as little code as possible. (In most cases this process will reveal the answer to you before you even post the question.)
This isn't to say more open-ended and subjective Stack Exchange sites don't exist. The English Language and Usage site, the Parenting site, etc. are all much more subjective in the questions and answers they accept. So, for any given site, you'll definitely want to check the FAQ and browse some example questions before proceeding. But for the technical sites (Stack Overflow, Server Fault, Super User, etc.) the format is generally much more concrete.