Cisco
interface Dialer1 (or FastEthernet0)
 ip nat outside
interface Vlan 1
 ip nat inside
ip nat inside source list TONAT interface Dialer1 overload
ip nat inside source static 192.168.1.2 203.87.57.205
 
Port forwarding:
ip nat inside source static tcp 192.168.1.2 25 203.87.57.205 25 extendable
ip access-list extended TONAT
 permit ip 192.168.1.0 0.0.0.255 any
 
Thoubleshooting:
show ip nat translationsclearip nat translation * 
sh ip nat statistics