i could be picking you up wrongly here , but
if you are saying for instance you forwarded port 59 to the IP 192.168.1.2 , then by default those ports are already assigned to that Ip

It's like with DCC Chat/send if i have 2 PCs 192.168.1.2 and i forward port 4000-4004 to it (then those ports are taken)
i cannot use them for another PC
for my next client for DCC to work 192.168.1.3 i need to select different ports like 4005-4009

perhaps this is where you are being misled

ScoT
#mIRC Undernet