You could write zeros to the whole hard drive
dd if=/dev/zero of=/dev/hda
For newer versions of dd, you can monitor the progress:
dd if=/dev/zero of=/dev/hda status=progress
Once you have decided to encrypt your hard drive, you might find yourself in the situation where you need to pull off data from it when the regular decryption mechanisms you use are not in place. E.g. when the OS is not booting or the drive has some physical problems. In that case you need to boot from a CD/DVD that supports the encryption technology that you used. In my case I need a live Ubuntu CD/DVD, preferably the same version that was used to encrypt the disk! Otherwise there might be incompatibilities:
sudo apt-get install lvm2 cryptsetup
In some networks, computers have to be registered to get connected. Upon registration, the computer will receive a name and an IP address. Every time it connects to the network, the DHCP server assigns it the same name and IP address. The DHCP server identifies computers based on the MAC address, which is unique to every NIC. However, if you know the MAC address of a registered computer, you can use it to connect to the network, faking the MAC address: