Single User licences
A Single-user license is assigned to one person and can only be used by the registered user. That individual can install the license on two computers that they use. This is typically a work computer/Desktop and a laptop.
With a single-user license, SketchUp only needs to ping the Internet to validate the license information and provide a license file every 28 days at the very least but SketchUp will always try to connect to the internet through Bing
Network licences
A network license is not assigned to an individual user. The software can be installed on as many computers as required. The license is stored in a Trimble-operated server in the cloud, and the software points to that license.
For version 2015, you no longer have to install a network licence on a server.
You install SketchUp on your workstation, and as soon as you authorise Sketchup with the licence details, it pings to the Trimble server/cloud which recognises it as a network licence. Then just repeat for all workstations. The Trimble Cloud stores the number of seats available on that network licence and will allow that amount of users access to SketchUp.
The 2015 licence is now installed just like a single user licence - although there is one important caveat to allow the authorisation to be successful: Ports 5053 and 50530 must be open on the router on each workstation for your network license to work. No server installs necessary.
For further information on licensing SketchUp, please see the below link:
http://help.sketchup.com/en/article/38589