You can only have 1 extended partition on a disk.
Because of the way it was all designed, way back when 20Mb hard drives were considered huge. You can only have 4 partitions on a drive, but one of these can be an extended partition which can hold logical partitions.
So it would go like this
Code:
sda1 primary
sda2 primary
sda2 primary
sda4 extended
\sda5 logical
\sda6 logical
\you can then carry on, there is a maximum but i can't remember how many.