P2P and NAT - How do I get through?

Hi. I'm building P2P network and wondering how do I get through NAT, to be more precise how to connect peer that is behind NAT to another peer that is also behind NAT. My network will also have peers that have a public addresses and I want (don't how it is possible) use them to establish connection between NATed peers.

What I know: How to establish connection between NATed and non-NATed peer (NATed peer ---> non-NATed peer)

What I do not know: How to establish connection between non-NATed and NATed peer (non-NATed peer ---> NATed peer). How to establish connection between NATed peers (NATed peer ---> NATed peer)

Thank you in advance. :)