I would connect two Moodle servers, firts in 1.9 verison and secon in 2.5.
I've activated Network in both servers.
When I add host in moodle 2.5 (Site administration/Networking/Manage peers), the host is added without problem.
When I try to add host un moodle 1.9, I get the next error:
"There was a problem retrieving the public key.
Maybe the host does not allow Moodle Networking or the key is invalid."
If I try to add manually, withe public key, the error is:
The key is not a valid SSL key.
4: The subject of the certificate: http://nameserver/moodle does not match the host it came from