I'm guessing "Fing" is a program to check for open ports? What I'm wondering is why it see's port 123456 when that's not even a real port number?
If it is seeing port 12345, that's likely the port the FreeNAS jail is using to receive RPC calls to do stuff like start and stop the plugin.
Yes sorry it is: http://www.overlooksoft.com/fing
nice free with premium features software on multiple platforms for scanning the network. If you have either iphone or android you can download it and see for your self. Unless it is an isolated case.