In basic NAT, we have the source IP address, IP header checksum (and any higher level checksums that include the IP address) of the outgoing packet are replaced with another IP address and the reverse translational takes place on a packet intended for this machine.
From Network address translation (Wikipedia):
..Basic NATs can be used to interconnect two IP networks that have incompatible addressing..
Basically NAT is used to connect a host on a private network (using private IP address) to the Internet (public network).
Can anyone share a practical scenario where basic NAT would be required to interconnect two IP networks that have incompatible addressing?