1. Navigate to Firewall | NAT.
2. Click on the 1:1 tab.

3. Click on one of the Add buttons to add a new rule.
4. In the External subnet IP text field, enter the public IP address:

5. Enter the private IP address in the Internal IP text field. Leave the drop-down menu set to Single host.
6. Leave Destination set to Any.
7. Enter a brief description in the Description text field.
8. When you are done making changes, click on the Save button.
9. Click on the Apply Changes button.

By adding a 1:1 NAT entry, we can translate an external IP address (or network) to an internal one.