Disk parition and mount

show all the partitions on hard disk
sudo fdisk -l


Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000db6bb

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1       18249   146585061   83  Linux
/dev/sda2           18250       36485   146480670   83  Linux
/dev/sda3           36486       54721   146480670   83  Linux
/dev/sda4           54722       60802    48839073    5  Extended
/dev/sda5           54722       55207     3903763+  82  Linux swap / Solaris
/dev/sda6           55208       60802    44935168   83  Linux

This is an example

For this hard disk. I only installed Debian on sda1. installed Ubuntu on sda6.
sda5 is the swap patition. I setted as logically.

I also partitioned sda2 and sda3. I set ext3 journaling format.
my previous blog has more detailed installation information.
I don't understand why sda4 came out. 

sda1 installed Debian
You could check OS version.

$ cd /mnt
$ sudo mkdir sda1
$ sudo mount /dev/sda1 sda1
$ cd sda1
$ cat /etc/lsb-release or /etc/redhat-release
e