Ubuntu as router with multiple WAN IP's on same subnet

Ubuntu as router with multiple WAN IP's on same subnet

at work we have 15 different WAN IP's. I'm trying to have a PC with Ubuntu
Server 12.04 installed and be able to NAT from the different WAN IP's into
our local network. Thing is that it all works fine with just eth0 and eth1
configured normally. But when I start to add multiple IP's such as eth1:1
and so on the router stops working. I'm using iptables and now I'm kinda
stuck here...
Anyone?
Best regards, Johnny Olaussen Norway