Cisco - Port Two Public IP Addresses to the Same Internal Address
Today we required the ability to port forward two public IP addresses both listening on TCP25 to the same internal IP address listening on TCP25. By default a Cisco router will not let you do this. However there is an extenable option which you can put on the end of your command to allow you to do this.
For example to allow TCP25 from both 188.8.131.52 and 184.108.40.206 to 10.1.1.40 on TCP25 we would do the following:
perth-router(config)#do show run | in ip nat ip nat inside source static tcp 10.0.8.10 25 220.127.116.11 25 extendable ip nat inside source static tcp 10.0.8.10 25 18.104.22.168 25 extendable