Can’t identify the URL shortener of those url’s [closed]

When I’m browsing my favorite forum, there is a guy who send a lot of resources about Security/Networking etc. Now, when I am pointing with my mouse on his link, I can see in my browser that the link is different than the actual url of the website. It’s shows that:<random_string> # a random string 

I’m searching the web for any information about this (url shortener?), and I just don’t find anything. Do you know anything about it ?