What is CUPS on port 631?
From the CUPS website:


  CUPS is the standards-based, open source printing system developed by Apple Inc. for OS X and other UNIX®-like operating systems.


On OS X it provides local printer services as well as shared printer services for other Macs to access your printer via the service if the printer doesn't have its own sharing capabilities. CUPS has become fairly ubiquitous across the Linux/UNIX contingent of OSes as the print server if choice.

That open port you found is because printer sharing is turned on on your machine. You can close port via System Preferences &gt; Sharing and uncheck Printer Sharing.