Multiple SNAT source address ranges

[Kirk Hoganson <kirk2@lenderlab.com>]

I am attempting to create SNAT rules that will round-robin through a 
series of addresses from different ranges as per the man page. 
Apparently I am making mistakes with the syntax, as running my script 
results in some errors.  Can someone tell me how I need to structure 
these rules?  Here are some examples of what I have tried:

$IPTABLES -t nat -A POSTROUTING -o $EXT -m state --state NEW -p tcp 
--dport 80 -j SNAT  --to-source 24.28.18.56 --to-source 24.28.18.57 
--to-source 24.28.18.58

Or:

$IPTABLES -t nat -A POSTROUTING -o $EXT -m state --state NEW -p tcp 
--dport 80 -j SNAT  --to-source 24.28.18.56-24.28.18.63 --to-source 
16.7.15.32-16.7.15.39 --to-source 16.7.87.64-16.7.87.71

How do I specify more than one source address from seperate ranges?