apt-get install without debian CD

For a fresh Debain installed from a CD, if someone wants to use apt-get install some packages, you may get the following error:

'Debian GNU/Linux 5.0.2a _Lenny_ - Official i386 CD Binary-1 20090817-00:24'
in the drive '/cdrom/' and press enter

if you have taken out the debian CD for a long time.
It is because that by default CD installation, apt-get will always check if your desired packages can be found in cdrom at first. Solution is simple, comment out the first item in your /etc/apt/sources.list

#deb cdrom:[Debian GNU/Linux 5.0.2a _Lenny_ - Official i386 CD Binary-1 20090817-00:24]/ lenny main