This method will create a wired network segment with only two hosts. Synergy will talk over the wire, while all the computers' other network traffic will use their default interfaces. It requires that your desktop have two ethernet interfaces.
Suppose you have a desktop, Ferris, with ethernet interfaces eth0 and eth1. It uses eth0 to connect to your workplace's wired network. Ferris will run Synergy Server.
You have laptop, Cameron, with ethernet interface eth0 and wireless interface wlan0. Cameron will run Synergy client.
Connect Ferris's eth1 to Cameron's eth0 with a network cable. Assign both interfaces static IP addresses in the same subnet. Don't specify DNS servers or default gateways.
For example, Ferris's eth1 gets IP address 192.168.100.1 and Cameron's eth0 gets 192.168.100.2. On both, the subnet mask is 255.255.255.0.
Configure Synergy according to your needs.
On Ferris, start Synergy Server with a command like this: synergys -c <path to your synergy config file> -a 192.168.100.1
On Cameron, start Synergy like this: synergyc 192.168.100.1
This ensures that Synergy only communicates on the little wired network that Ferris shares with Cameron.