Multiple ISP Public IPs over a single circuit

[ManInTheMiddle]

HI

I have 2 public IPs coming over a single circuit terminating in a firewall (Cisco ASA 5525)

One is for a SIP trunk and one is for Internet connectivity.

I am wondering how to configure the firewall interface to use both Public IPs on the single interface. Would it be sub-interfaces? 

[balaji.bandi]

You do not required to configure IP address to interface, if they are routed to your FW interface, you can do NAT based on the requirement.

if this was not the case explain, how they routed ?

[ManInTheMiddle]

I am not sure how the ISP are routing but originally it was a single public IP for internet connectivity to the FW interface and we had to add second public IP to that link for a SIP trunk. 

I think NAT the second IP for the SIP trunk is the way forward.

Thank you for your reply.  

[Marius Gunnerud · Accepted solution]

You can use proxy ARP with NAT, then you will not need to configure the IPs on the physical interface.  Optionally you could configure the IP for internet traffic on the interface and NAT the SIP IP

[ManInTheMiddle]

This is exactly what I need to do. My confusion was coming from the physical interface being originally configured with a public IP just for Internet and additional IP added for SIP trunk and had me scratching my head how both Public IPs will come into that interface. 

Thanks for the reply.