Programming Network subsystem

["Mohammed Awad" <MAwad Enppi com>]

Hi all,
I need someone to tell me about some good reference to push me into the field of programming of the Linux Network subsystem. I need to write a code to implement some additional functionality for the NAT protocol. This is a part of my post graduate research. I didn't practice C for enough period of time, only have rather fair basics. Also I have gained fair knowledge base of the concrete architecture of the Linux. Now I need to proceed directly twards my point. I need some reference that is my focus. Any help is highly appreciated.
Thanks 
Mohammad Awad