/0     0.0.0.0
/1     128.0.0.0
/2     192.0.0.0
/3     224.0.0.0
/4     240.0.0.0
/5     248.0.0.0
/6     252.0.0.0
/7     254.0.0.0
/8     255.0.0.0

/9     255.128.0.0
/10    255.192.0.0
/11    255.224.0.0
/12    255.240.0.0
/13    255.248.0.0
/14    255.252.0.0
/15    255.254.0.0
/16    255.255.0.0

/17    255.255.128.0
/18    255.255.192.0
/19    255.255.224.0
/20    255.255.240.0
/21    255.255.248.0
/22    255.255.252.0
/23    255.255.254.0
/24    255.255.255.0

/25    255.255.255.128
/26    255.255.255.192
/27    255.255.255.224
/28    255.255.255.240
/29    255.255.255.248
/30    255.255.255.252
/31    255.255.255.254
/32    255.255.255.255

Additional:
From: http://www.digipro.com/Papers/IP_Subnetting.shtml 

Subnet Masks and Subnets:

notation		resulting subnet
netmask	shorthand	number of addresses
255.255.255.0	/24 [8-bit]	28 =	256	= 254 hosts + 1 bcast + 1 net base
255.255.255.128	/25 [7-bit]	27 =	128	= 126 hosts + 1 bcast + 1 net base
255.255.255.192	/26 [6-bit]	26 =	64	= 62 hosts + 1 bcast + 1 net base
255.255.255.224	/27 [5-bit]	25 =	32	= 30 hosts + 1 bcast + 1 net base
255.255.255.240	/28 [4-bit]	24 =	16	= 14 hosts + 1 bcast + 1 net base
255.255.255.248	/29 [3-bit]	23 =	8	= 6 hosts + 1 bcast + 1 net base
255.255.255.252	/30 [2-bit]	22 =	4	= 2 hosts + 1 bcast + 1 net base
255.255.255.254	/31 [1-bit]	21 =	-	invalid (no possible hosts)
255.255.255.255	/32 [0-bit]	20 =	1	a host route (odd duck case)