Assume that we have a file server that must be accessible from the internet. We have a public IP address, which we obtained from our ISP. We follow this procedure:

1. Navigate to Firewall | NAT, and click on the 1:1 tab. On the 1:1 page, click on one of the Add buttons.
2. On the Edit page, add the External subnet IP, which in this case would be just a single IP address.
3. For Internal IP, select Single host in the drop-down box and enter the internal IP address. Click on the Save button when you are done. Click on Apply Changes on the 1:1 page.

This procedure can be used to map a single external IP address to a single internal IP address, or you could map a block of IP addresses by using the external block’s subnet mask and mapping it to the internal block’s subnet mask, as outlined in the description of 1:1 NAT previously. When you are done mapping the IP addresses, you will want to test the mapping by accessing the file server from the other side of your firewall. In such cases, a separate internet connection such as a mobile connection can be helpful.