Basic NAT and PAT

There are two levels of network address translation.
• Basic NAT. This involves IP address translation only, not port mapping.
• PAT (Port Address Translation). Also called simply "NAT" or "Network Address Port Translation, NAPT". This involves the translation of both IP addresses and port numbers.
All internet packets have a source IP address and a destination IP address. Both or either of the source and destination addresses may be translated.
Some internet packets do not have port numbers. For example, ICMP packets have no port numbers. However, the vast bulk of internet traffic is TCP and UDP packets, which do have port numbers. Packets which do have port numbers have both a source port number and a destination port number. Both or either of the source and destination ports may be translated.
NAT which involves translation of the source IP address and/or source port is called source NAT or SNAT. This re-writes the IP address and/or port number of the computer which originated the packet.
NAT which involves translation of the destination IP address and/or destination port number is called destination NAT or DNAT. This re-writes the IP address and/or port number corresponding to the destination computer.
SNAT and DNAT may be applied simultaneously to internet packets.
NOTE: 'PAT', as it is referred to here, is referred to by Cisco as NAT 'overloading', as described in this Howstuffworks article, provided to Howstuffworks by Cisco: http://computer.howstuffworks.com/nat3.htm