Synergy 2 uses your local area network (LAN) to send keystrokes, mouse movements and clipboard data between computers. This requires at least one of your computers to see the other, so they can talk directly. However, what if one of your computers is on an isolated WiFi network, or your Ethernet LAN is locked down so your computers can't see each other. Well, there's a couple of things you can do...
Using 2 USB Ethernet adapters and a crossover cable, you can connect your two computers together to make your own private Ethernet LAN. You can do this on Windows, macOS, and well, Linux users... you know what you're doing.
Using 2 USB WiFi adapters, this is the solution for those who hate wires. However, Microsoft has made it a little trickier to do this since Windows 10. On macOS, it's a little easier (no surprises there), and Linux, again, you guys know your stuff.
A little-known trick, available only for macOS users, it's actually possible to connect two Macs together using a Thunderbolt cable!
We don't recommend this, but one very tempting solution is to use Bluetooth by setting up and ad-hoc Bluetooth PAN. This was very popular before the advent of WiFi and used to be quite well supported for lightweight stuff like sending an email. However, when it comes to highly sensitive mouse-based apps like Synergy, most people seem to report terrible lag developing over time caused by poor quality Bluetooth drivers or hardware. You're better off with one of the solutions above (Ethernet, WiFi or Thunderbolt).