There seems to be something wrong with the interfaces you are using, as the iptables command is correct.
-t nat = apply this command in “nat” table.
-I POSTROUTING 1 = install this command in POSTROUTING chain in line 1
–src x.x.x.x/yy = the source IP is x.x.x.x/yy
-o ETH00 = the output interface is “ETH00”
-j MASQUERADE = masquerade the source IP with the IP of the interface ETH00