# Defining address translation

The NAT cell of the NAT rules allows you to define that the source address, the destination address, or both addresses are translated..

If a connection matches the rule, the address translation defined in the NAT cell is applied. You can leave the NAT cell empty if you do not want to apply NAT to any connections that match the rule.

Static translation creates a one-to-one relationship between the original IP addresses and the translated IP addresses. When you use static source or destination translation, the translated address space must be as large as the original address space.

Dynamic source translation creates a many-to-one relationship between the original IP addresses and the translated IP addresses, so that several hosts can use the same IP address. In dynamic translation, a port is reserved for each host that is communicating. The number of ports in the port range determines how many hosts can communicate simultaneously using a single IP address. If the number of hosts exceeds the number of ports in the port range, translation cannot be applied and some of the communications fail. If failures happen, you might need to divide the dynamic translation rule and use an extra IP address for the translation. Dynamic translation can only be applied to communications that use ports (TCP and UDP-based protocols).