The following yEd features try to open a network connection.
New Version Check
On every startup, yEd tries to connect to the Internet to check if a new version is available on the yWorks server. Therefore, it loads a small text file containing the version number of the most current release.
If a new version is available, yEd displays a notification dialog with a Visit yEd Homepage button. Clicking this button opens the yEd Homepage in an Internet browser.
This check can be disabled in yEd's preferences.
Open Files in Existing Window
Additionally, yEd checks on startup whether it is already running on this computer. This is done to open graphs which are, for example, double-clicked in Explorer in an already running yEd window instead of starting yEd several times. To this end, yEd tries to connect to an already running other yEd using a socket. Roughly speaking, a socket is an internal network connection which may be interpreted as an attempt to connect to the Internet by your firewall. However, no Internet connection is opened by this feature.
As of version 3.12, yEd uses a local socket for this feature. I.e. the socket is restricted to inter-process connections on this computer.
In order to prevent this behavior, you can start yEd with the option -nosingle from the command prompt. If you're using Windows and installed yEd with an embedded JRE in the default location, you can use a command like the following:
C:\Program Files\yWorks\yEd>start jre\bin\javaw -jar .\yed.jar -nosingle
When using this feature for the first time, yEd connects to https://www.yworks.com/ and downloads the most recent One-Click Layout binaries. On each subsequent startup, yEd connects to https://www.yworks.com/ and checks for new One-Click Layout binaries. If new binaries are available, yEd automatically downloads the new One-Click Layout binaries.
Each time One-Click Layout is used to arrange a diagram, yEd will open an Internet connection to https://www.yworks.com/ and upload an anonymized version of the diagram along with user feedback and an ID for identifying diagrams sent from the same yEd installation.
An Internet connection is opened to import images from the IconFinder icon search engine.