Postfix ip rotation

[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.180
[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.181
[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.182
[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.183
[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.184
[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.185
[root@mail2 ~]# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth1 -m statistic --mode nth --every 7 -j SNAT --to-source  192.168.140.186

for 7 ip address 180-186

change --> every _#num of ip__