How to install broadcom 4321 on Ubuntu 12.04

Driver from lspci: Broadcom Corporation BCM4321 802.11a/b/g/n (rev 03)

On Ubuntu, you will need headers and tools. Try these commands:
sudo apt-get install build-essential linux-headers-generic
sudo apt-get build-dep linux

sudo apt-get update
sudo apt-get –reinstall install bcmwl-kernel-source

Reboot and try it out!