磁盘分区疑问

[root@lzf home]# fdisk /dev/sda

Command (m for help): m
Command action
a toggle a bootable flag
b edit bsd disklabel
c toggle the dos compatibility flag
d delete a partition
l list known partition types
m print this menu
n add a new partition
o create a new empty DOS partition table
p print the partition table
q quit without saving changes
s create a new empty Sun disklabel
t change a partition's system id
u change display/entry units
v verify the partition table
w write table to disk and exit
x extra functionality (experts only)

Command (m for help): p

Disk /dev/sda: 21.5 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders, total 41943040 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000d10ae

Device Boot Start End Blocks Id System
/dev/sda1 * 2048 1026047 512000 83 Linux
/dev/sda2 1026048 41943039 20458496 8e Linux LVM （这里应该是表示我的磁盘只分两个主分区吧）

Command (m for help): n
Command action
e extended
p primary partition (1-4)
e
Partition number (1-4, default 3): 2
Partition 2 is already defined. Delete it before re-adding it.

Command (m for help): n
Command action
e extended
p primary partition (1-4)
e
Partition number (1-4, default 3): 4
No free sectors available

Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4, default 3): 3
No free sectors available

Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4, default 3): 4
No free sectors available

Command (m for help):
就到这里了，为什么会No free sectors available 上面显示是只有两个分区啊，不是应该可以最多创建4个分区吗？4个主分区或者4个主分区加一个extended。