man iptables --to-source ipaddr[-ipaddr][:port[-port]]
which can specify a single new source IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid
if the rule also specifies -p tcp or -p udp). If no port range is specified, then source ports below 512 will be mapped to other ports
below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be mapped to 1024 or above.
Where possible, no port alteration will
In Kernels up to 2.6.10, you can add several --to-source options. For those kernels, if you specify more than one source address, either
via an address range or multiple --to-source options, a simple round-robin (one after another in cycle) takes place between these
addresses. Later Kernels (>= 2.6.11-rc1) don't have the ability to NAT to multiple ranges anymore.