Duet connections will work over Thunderbolt cables using IP over Thunderbolt. Thunderbolt direct connections are fast and reliable; an excellent way to run Duet between two machines.
You'll need two Macs with compatible Thunderbolt connections, and a Thunderbolt cable. There are various revisions of Thunderbolt; you'll need to make sure that the cable you have is compatible with both Macs. Depending on the vintage of your Mac, it may support only the original Thunderbolt, Thunderbolt 2, Thunderbolt 3, or even Thunderbolt 4. See the technical specs for your Mac models to determine which kind of Thunderbolt cables they support.
Plug one end of the cable into each Mac, making sure you choose a Thunderbolt-capable port. Thunderbolt cables are typically USB-C, but not all USB-C ports on your Mac necessarily support Thunderbolt; look for a little lightning icon over the port.
You can plug the cable in while the Mac is running; the cable should be detected and activated by macOS automatically.
Once the two Macs are connected by the Thunderbolt cable, open the macOS System Preferences.
Select the network settings:
You should see the network settings:
The "Thunderbolt Bridge" is the connection over the Thunderbolt cable. If you do not see this, macOS has not detected your Thunderbolt connection, and you may need to change the cable or the port in which it is inserted.
By default, the Thunderbolt Bridge will come up unconfigured, with a red dot. After a short delay (and once both Macs are connected), macOS will assign it a "Self-Assigned IP", and display a yellow dot. The Thunderbolt cable is usable at this point, and Duet will use it automatically.
You're ready to go!
If you wish, you may manually supply an IP address to the Thunderbolt Bridge. If you do this, it needs to be done on both Macs, and they must have different IP addresses that are in the same netblock. To do this, use the menu connected to "Configure IPv4:".
You will need to fill in both the "IP Address" and "Subnet Mask" fields. The addresses should not be in the same netblock as any of the other networks listed in the Network panel of System Preferences.
Once both Macs have IP addresses with corresponding subnet masks, the dot should appear green.
If you wish to test whether the Thunderbolt direct link is active, open a command line terminal such as Terminal.app and use the 'ping' command with the Thunderbolt Bridge address of the other Mac:
If you see 'Request timeout for icmp_seq' or more than 0% packet loss, there may be an issue with your Thunderbolt cable, it may not be plugged in correctly on the remote system, or you may have entered the IP address wrong in the 'ping' command.
Once the Thunderbolt direct link is available, Duet will use it automatically the next time a Duet connection is started.