Originally Posted by dave247
The area I am confused about is how transmissions are getting sent between nics
Well, that's something you learn when studying networking basics. Seems to me, you should consider reading something like this:
, old but some parts are still pertinent. Or this: http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch03_:_Linux_Networking
, newer info. And some "Networking 101" books. There are some CCNA books that are pretty good, IMHO.
Anyway, a quick-and-dirty answer: The way in which packets are transferred between different NICs (between different networks, that is) is called "Routing". You configure your gateway with some info, called "routing tables", which are used to decide where a packet should go. For example, one packet wants to go to your LAN, then your gateway routes it through the NIC that is connected to the LAN. If another packet wants to go to the Internet, then your gateway routes through your ISP connection.
If what you want is getting your LAN to access the Internet through your gateway, then yes, you could use NAT/Masquerading