Dropbox works by uploading your files to the internet, and then downloading them to each of your devices. This works fine most of the time, but what if you’ve got a large file—say, a video—on your PC, and you want to transfer it to your laptop? This could easily take a few hours, depending on the speed of your connection.
But when the file is on your Local Area Network (LAN), syncing is much, much faster. With LAN Sync, your file still has to upload to the Dropbox server before it can sync, but it does not need to be downloaded to each device. So, once the file is on the server, two computers will update from each other if they’re on the same LAN.
To switch it on, right-click on the Dropbox icon in the Taskbar and click Preferences.
Then click Enable LAN sync. It is enabled by default, but will be disabled if your Firewall blocks it. The solution to this is to turn off your firewall, enable LAN sync, and then turn your firewall back on, making sure that Dropbox is allowed through.
More information on the more technical aspects of LAN Sync is available at www.dropbox.com/help/137/en.
Updated on 1/21/15 with clarified and corrected information that implied you didn’t need to upload documents to Dropbox server in order to use LAN sync.