Netmask Table

/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)