Adding a Second Public IP Range

Question

Our current environment is configured as follows and is in the attached image.  We have two routers on the front end that are running BGP with three different ISP's.  Each ISP connects to a different interface on our routers.  The ISP interfaces use ISP supplied /30 IP's.  Our existing ARIN IP block is advertised out BGP and assigned to the inside interface of each router.  We run iBGP and GLBP on the routers inside interfaces.  We are close to using up our /24 block of ARIN assigned IP's.  We have received a new /24 block of ARIN assigned IP's that we want to use along with the existing /24 block.  What is the best way to add the new /24 block of IP's to our routers?  Do I need to assign them as a second IP to each router's inside interface?  Or, can I just advertise the new block out of BGP and add a static route to route the new block out the inside interfaces and not assign any secondary IP's to the inside interfaces?

Answer 1

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