OPC DA and OPC UA servers are discovered through the same methods.
DA servers are enumerated using the OpcEnum enumeration server (part of the OPC Core Components)
UA servers are discovered through an UA Discovery server. The UA discovery access details are defined in the application UA configuration.
A Local UA Discovery Server must be running for the UA server discovery to be possible. UA discovery servers report the UA servers running on the same machine.

OPCDA.NET-UA enumerates both type of servers. UA servers are identified in the enumeration result by the ua: prefix.
This is as UA servers need to be identified in the server access.

Server on networked machines can be discovered by specifying the network machine name. This works for the discovery of DA and UA servers.
Machines outside the local network cannot be accessed with DCOM and classic OPC DA can neither be discovered not accessed. To discover UA servers on such machines the machine name must be prefixed with  ua:   e.g.  ua:opcua.info
This disables the enumeration of DA servers.
Specify ua: as the machine name to browse only UA servers on the local machine. Use this e.g. if OPC server browser OpEnum.exe is not installed.

The machine name prefix da: can be used to restrict browsing to Classic OPC DA servers. The UA discovery server is not accessed in this case.