Inside source -> Translates the source IP addresses in packets from inside to outside network, and translates the destination IP addresses in packets from outside to inside network.
The concept of Port Forwarding is easy. Assume that I have an ADSL modem/router for my home Internet access, it public address is x.50.123.123, and its private address (acting as the gateway for the internal network) is 192.168.0.1.
There is a HTTP server inside the home network, with an IP address of 192.168.0.2. Assume that I would like to let one of my friends access and evalute my homepage, here I will configure port forwaring of the ADSL router with a rule as: "If someone over the Internet (public network) accessing HTTP service (Port 80 in this case, we can configure this to be any number with the global-port-num parameter in the configuration portion below) at x.50.123.123, I shall direct the traffic from the requesting host (HTTP client) to the HTTP server, and passes (NAT) the replies / traffic from the HTTP server back to the HTTP client over the public network.
