WS-Discovery (Web Services Dynamic Discovery) is the mechanism ONVIF uses to find cameras on a local network automatically. When a VMS scans for devices, it sends a multicast "probe" message on the LAN; ONVIF cameras that hear it answer with their address and capabilities, so the operator sees a list of discoverable cameras instead of typing in IP addresses one by one. It is the standard that makes "scan and add" onboarding possible.
Mechanically it is a SOAP-over-UDP multicast protocol: the probe goes to a well-known multicast group, and responses come back directly. This is why discovery is so convenient on a flat local network — and why it has a hard boundary. Multicast typically does not cross subnet or VLAN boundaries or traverse routers without help, so cameras on a different network segment from the VMS often will not appear in a WS-Discovery scan even though they are reachable by IP.
The pitfall, then, is relying on auto-discovery in a segmented or large network. Best-practice surveillance VLANs deliberately isolate cameras, which breaks multicast discovery across segments; the fix is to add cameras by IP range or hostname, or to use the VMS's bulk-onboarding tools, rather than expecting one flat scan to find everything. Discovery is for finding devices, not a substitute for an IP addressing and onboarding plan at scale.
