Configuring Network Address Translation (NAT) on a device might be required for these purposes:
To hide the actual IP address of the inside hosts
To strictly control the ability of the inside devices to access the outside world
To strictly control access to inside hosts from the outside world
NAT overload or Port Address Translation (PAT) allows multiple internal addresses to be translated to an external address by additionally using port numbers to distinguish between translations. When a device configured for NAT overload receives Internet Control Message Protocol (ICMP) fragments, the manner in which the fragments are handled depends on the order in which fragments are received and the state of the NAT translation table.