Make a specific content-type inaccessible I have a content-types that only is linked from nodes of other content types and therefore should not be directly accessible. In other words, I don't want a visitor to be able to access these kind of nodes by going to node/&lt;nid&gt;. Is this possible?
Yes, the Rabbit Hole module was built for exactly that:

Rabbit Hole is a module that adds the ability to control what should happen when en entity is being viewed at its own page.
Perhaps you have a content type that never should be displayed on its own page, like an image content type that's displayed in a carousel. Rabbit Hole can prevent this node from being accessible on its own page, through node/xxx.

It's highly configurable and works seamlessly in my experience.