Hi
I'm very sure that you have some preferences settings for a proxy in Eclipse (Window->Preferences->Proxy). Assuming that you dont need a proxy then clearing those will prevent this dialog from popping up.
If you need a proxy then things get more complicated. A bad design in the JDK, a very lacking default proxy plugin and several competing fixes cause Eclipse to behave differently depending on the plugins you have. The dialog you screenshotted is caused by the default Eclipse proxy support in org.eclipse.ui.net. It's limitations are very well known but none fixed those in the platform yet:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=196780
This very lacking plugin is overriden by EGit, Scout and others. Depending on which of these plugins you have, you will or wont get the above dialog. The best advice I can give you is to make sure that you have EGit installed (and make sure it's started: have some git shared projects in your workspace or open the "Git Repository Exploring" perspective" at least once). EGit overrides the default proxy plugin by a far superior plugin that prevents the above dialog from appearing.
If you need more details, the you can have a look at a very similar issue we ran into in OpenShift/JBoss Tools. The jira issue holds all informations and insights I collected when looking into this weirdness:
https://issues.jboss.org/browse/JBIDE-12999
Cheers
André