Bella Hadid's 21st birthday celebration went down exactly as you'd like it to -- ass out!

Bella went bottoms up in a g-string to make sure all her fans could, umm ... toast her big day. Then she put on something more dinner-appropriate for a night out in NYC with her mom, Yolanda, and Gigi.

Even when they're just walking out of a building these ladies look like they're working the hell out of a runway. Happy 21st, Bella!

Your apple did NOT fall far from the tree.