I used to have a link to an excellent pdf that showed that IP Communicator used 3 DLU's as Majid has nicely noted :) Of course this link no longer works so I am attaching a different pdf (see page 18).
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.