Custom Wordpress outbound link?

[greenshoes]

I'm sure they use full page script for outbound links, but how they manage to make the URL look like this... ???

Someone care to teach me how to do so

 

[Tennouji]

I think it's just encoding the real URL to base64 stringThere are plugins for that, though I saw plugins doing url masking better than that since someone can just decode the base64 string to see the real url.