Many bloggers talked about first by server exchange among A, B A - NAT, B - NAT address, and then A: port1 through A - NAT: port1 sent to B - NAT: PORT2, for the first time will be discarded, but can receive B by B - NAT from information, but in fact limited port type NAT must ensure that the packet source address and port is has been previously we have already send the destination address and port,
That is:
A: port1 - A - NAT: port1 "======& gt; B - NAT: PORT2 is first B - NAT discarded;
So:
A - NAT: PORT1 can only receive B - NAT: PORT2 from data!
But:
In fact, as a NAT network CARDS, we in writing the program, can only be bound B: PORT2, NAT should not automatically mapped to B - NAT: send PORT2 port, so, that is to say, may through the B - NAT: PORTX, cannot ensure success make hole?

To sum up, as if only completely cone type and limited NAT can make hole is successful, the rest seem not to can? But online only says that symmetrical type can't!