I just had this request from my IT department. They are running out of public IPs and the ISP just gave them a new range (they will keep using the old range). They are using ASA 5510 as their gateway device. I am wondering if I can create a sub-interface on the ASA eth0 (however doesn't belong to any VLAN, just untagged as the main interface) and assign it another public IP (as well as another default gateway). Will this setup work? Any advice is welcome. Thank you!
It will not work.
Have the ISP send traffic destined to the new range towards you currant ASA ip address and NAT that range on the ASA's outside for inside hosts.
I hope it helps.