Network Address Translation (NAT) is used to redirect the traffic received by a public IP address, to an internal private IP address. This process is used by your router to direct individual port information for the public IP address into information that can be assigned to multiple internal private addresses.
If you need your network to be visible from the Internet and you wish to run a Web server, IP cameras, Mail server etc, then you need an IP address that is always visible. You could use a Single Static IP address and port forwarding. As most devices have different ports as standard or can be assigned ports, then a single IP address using port forwarding would allow you to run a WEB server, a Mail server, Remote desktop, IP cameras and more.
If you need to run more than one mail server and need the same port on both (port 25 etc) then you should consider using our No NAT range of Multiple Static IP addresses.
Need more IP addresses? see How do I order or add more IP addresses?