A subnet is a subnetwork, meaning a network within a network! We won't go into the nitty-gritty technical aspects, but ultimately this means that a network is divided into multiple smaller networks on different IP ranges. Roon requires all devices to be on the same IP range, so any devices used by Roon can't be on a different subnet than the server. This includes remotes and endpoints.
First, you need to determine the IP address of your devices. If you need help with this, please see the section below!
An IP range will be something like 192.168.1.x, and 'x' changes for each device on the network. For example, your server machine might be 192.168.1.4. If this is the case, you'll need your remotes and endpoints to also start with 192.168.1. If those first 3 sets of numbers are different, then they're on a different subnet!
There are a number of ways to find the IP addresses for your devices. You can check your router's UI, or use an app like Fing that scans the network. For OS-specific instructions, please choose the relevant option below:
For other devices, please refer to your product manual.