Yes it's normal. Rabbits go through a molt about four times a year.  Two heavy, and two light molts.  Usually a bunny's first molt is very heavy.    Most bunnies are going through a molt right now.  (of course it can depend on the weather too!)

During a heavy molt, it is a good idea to brush them once a day (but be gentle as their skin is sensitive) so they don't digest so much hair.  They can't cough up hair like cats can.

Also, make sure they have a high fiber diet as that's what's needed to process the hair through their system.    So lots of hay!

is there a special brush you have to use?

Jack and Rucy are finishing up their heavy molt, and Bailey is in the middle of hers.  Sometimes theirs actually stops in the middle and starts up again a week later.  I think it's because this Northern California weather can go between warm and cold extremes rather quikly in the spring.  Then they do it again in the late fall - October. 

So for our household - the heavy molts happen in April & October, but I do find that most of the bunnies at the rabbit rescue, saveabunny, also go through it at the same time.