Using a second Public IP Address:

[WhiskeyHughes]

Guys, 

My client has been allocated 2 Public IP Addresses from the ISP, but we always just used one of them because there was never a need to use the second. Well, we now have a situation, where we need to port forward port 5060 to two different local IP addresses. The obvious solution would be to use the second public IP Address and port forward it to 5060 and then call it a day. I just don't understand how that works on a cisco router. Does the router know about all of the Public IP's because when I added the main public IP Address on the WAN port, I put the subnet mask in that accounts for the 2 IP's? How do I make the Router aware of my second Public IP Address? 

Thanks in advance for all of your help,  

[Jon Marshall]

You won't be able to port forward the same port on the same public IP to different local IPs  because the router will have no way of knowing which local IP it is meant for.

But what you could do is use your existing IP for one of the translations and the second IP for the other eg.

ip nat inside source static tcp 192.168.5.10 5060 interface gi0/1 5060

where gi0/1 is the outside interface and then

ip nat inside source static tcp 192.168.5.11 5060 <unused public IP> 5060

In terms of how it works with the second public IP the ISP has a router with an IP from the same public IP range. So when traffic gets to ISP router inbound from the internet the ISP router requests the mac address of the public IP.

Your router performs proxy arp for that public IP so it responds with the mac address of it's outside interface.

This means that traffic for that IP is sent to your router.

Jon