The net mask can be written in decimal numbers or in binary numbers.
If you write it binary, you just tell how much 1's you see.
Try to make it clear like this:
 Code:
/20 = 11111111.11111111.11110000.00000000 = 255.255.240.0
/21 = 11111111.11111111.11111000.00000000 = 255.255.248.0
/22 = 11111111.11111111.11111100.00000000 = 255.255.252.0
/23 = 11111111.11111111.11111110.00000000 = 255.255.254.0
/24 = 11111111.11111111.11111111.00000000 = 255.255.255.0
/25 = 11111111.11111111.11111111.10000000 = 255.255.255.128
/26 = 11111111.11111111.11111111.11000000 = 255.255.255.192
/27 = 11111111.11111111.11111111.11100000 = 255.255.255.224
/28 = 11111111.11111111.11111111.11110000 = 255.255.255.240
/29 = 11111111.11111111.11111111.11111000 = 255.255.255.248
/30 = 11111111.11111111.11111111.11111100 = 255.255.255.252
/31 = 11111111.11111111.11111111.11111110 = 255.255.255.254
/32 = 11111111.11111111.11111111.11111111 = 255.255.255.255

172.16.0.0/27 means:
a subnet of the class B network 172.16.0.0
(default network mask = 255.255.0.0 or /16)
network address = 172.16.0.0
host adresses: 172.16.0.1 - 172.16.0.30
broadcast adress = 172.16.0.31

Here it is explained by someone else:
http://www.nongnu.org/lpi-manuals/lpi-102/html/ch05s02.html

[Edit]
Thanks Gargoyle, you hit the nail right on the head
[/Edit]
[Edit]
192.168.1.* would be 192.168.1.0/24
[/Edit]

Edited by Witto (2007-12-02 04:58 PM)