From the Shorewall documentation http://www.shorewall.net/Shorewall_a...nterfaces.html
The ifconfig utility is being gradually phased out in favor of the ip utility which is part of the iproute package. The ip utility does not use the concept of aliases or virtual interfaces but rather treats additional addresses on an interface as objects in their own right.
ip addr add 18.104.22.168/24 dev eth0
I use this method on a corporate firewall that has over 30 IP's with no problems.