Possible to Do Double NAT

Yes but that is bad designing and will lead to other problems.

This situation is very common. In that case the best way to do is: 1. Configure the VPN in mode interface. 2. Create a VIP for a virtual LAN (the ' virtual' subnet that peer is going to reach in the VPN). 3. For input traffic use the VIP (destination). 4. For output traffic use a regular firewall policy with NAT enable. The peer has to do the same for another virtual LAN. Regards

The FortiOS handbook has a few pretty good examples for exactly the same scenario. I' d suggest to start at section " How to work with overlapping subnets" . You can download the guide from http://docs.fortinet.com/d/fortigate-fortios-handbook-the-complete-guide

I' ve done this before and the only real issue I get is with DNS. If you have DNS forwarders to their DNS servers for things like server names and such, it will return the original IP address, which is on your subnet, not the translated subnet. I' ve had both sides setup a secondary DNS zone, company1.local, company2.local, on their side, so it has a different FQDN from their local naming scheme. Then manually populate the A records for important resources. Example: Server1: 192.168.101.5 Translatested to 192.168.201.5 Original FQDN: server1.company.net Translated FQDN: server1.company1.local This way you at Company 2 can use the FQDN of the translated item and if they switch the device, they just update their DNS. Also makes it so if you are going to map things like drive letters and such the IP addresses aren' t as visible to the end user. Users always questions why they have to use the IP address and not a name like everything else.

@Brady FortiOS has a " DNS translation" feature which might be of good use for you. When set up on a port, a DNS request is passed trough but the reply is translated to a different IP address which you specify. Have a look at the CLI Reference for starters. It' s one of the more rare features but might be quite helpful.

Thanks!! http://docs.fortinet.com/uploaded/files/800/fortigate-cli-50.pdf Page 109