One to May NAT

conf t
int eth0
ip address 192.168.1.1 255.255.255.0
no shut
!
conf t
int se0
ip nat outside
ip address 10.20.1.1 255.255.255.0
clock rate 64000
no shut
!
access-list 12 permit 192.168.1.0 0.0.0.255
ip nat inside source list 12 interface serial 0 overload
!
router rip
network 192.168.1.0
network 10.20.1.0