NAT (Network Address Translation)
NAT either assigns the client an unused port number (if only one public IP is available, as in ICS), or it assigns the client a mapping to a public IP address (if multiple public IP addresses are available). If multiple public IP addresses are available, they may run out at some point, in which case NAT begins assigning port numbers to identify internal network clients. Thus, you may have NAT setup with 10 public IP addresses and 20 private computers. The first 9 private computers would get mappings to the public IP addresses ; the next 11 private computers to come online would get port assignment.
If inbound traffic is not in response to prior outbound traffic, and it is not addressed to a specific private client by means of a static mapping, the traffic can't get into the private network. When you configure inbound connections, you are allowing requests that originate on the Internet to access your internal network.
