ip addr show dev eth2 | grep 192.1.33.254 || ip addr add 192.1.33.254/24 dev eth2
nic #
 ip addr show dev eth2 | grep 192.1.2.250 || ip addr add 192.1.3.250/24 dev eth1
nic #
 iptables -t nat -F
nic #
 iptables -F
nic #
 iptables -X
nic #
 # port for the first address
nic #
 iptables -t nat -A POSTROUTING -s 192.1.3.209 -p udp --sport 4500 -j SNAT --to-source 192.1.2.254:3503-3509
nic #
 iptables -t nat -A POSTROUTING -s 192.1.3.209 -p udp --sport 500 -j SNAT --to-source 192.1.2.254:2503-2509
nic #
 # for the second address use different port range
nic #
 iptables -t nat -A POSTROUTING -s 192.1.33.222 -p udp --sport 4500 -j SNAT --to-source 192.1.2.254:6503-6509
nic #
 iptables -t nat -A POSTROUTING -s 192.1.33.222 -p udp --sport 500 -j SNAT --to-source 192.1.2.254:5503-5509
nic #
 iptables -t nat -A POSTROUTING --destination 0.0.0.0/0 -j SNAT --to-source 192.1.2.254
nic #
 iptables -t nat -L -n
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination         
SNAT       udp  --  192.1.3.209          0.0.0.0/0            udp spt:4500 to:192.1.2.254:3503-3509
SNAT       udp  --  192.1.3.209          0.0.0.0/0            udp spt:500 to:192.1.2.254:2503-2509
SNAT       udp  --  192.1.33.222         0.0.0.0/0            udp spt:4500 to:192.1.2.254:6503-6509
SNAT       udp  --  192.1.33.222         0.0.0.0/0            udp spt:500 to:192.1.2.254:5503-5509
SNAT       all  --  0.0.0.0/0            0.0.0.0/0            to:192.1.2.254
nic #
 iptables -L -n
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
nic #
 echo initdone
initdone

