Probably, Need a router selecting.
My router make the source IP change to the WAN IP.

http://gyazo.com/3e7cb31756a9db04aa1bfcbff237ad0b
image mistake:
x"Factorio server:11111"
o"Factorio server:11112"

But . I think that other router is not necessarily the same.
NAT is Fucking. Compatibility of NAT ...

I use the NAT loopback enabled routers.

Example)
server-192.168.x.1:11111(local IP),globalIP:9.8.7.6:11111 (forwarding)
myPC-192.168.x.2:11112(loacl IP),globalIP:9.8.7.6:11112(forwarding)
playerA-1.2.3.4:12345(global IP)

If don't use NAT loopback. use the local IP to server connection.
can : myPC ...

MP mode can 2-4 people. cant 5~ people. yet
4~5th connect player black out and all other players freeze ;D