Subnet Cheatsheet:

/slash  subnetmask     usable
/23 = 255.255.254.0    (510)
/24 = 255.255.255.0    (254) 
/25 = 255.255.255.128  (126)
/26 = 255.255.255.192  (62)
/27 = 255.255.255.224  (30)
/28 = 255.255.255.240  (14)
/29 = 255.255.255.248  (6)
/30 = 255.255.255.252  (2)
/32 = 255.255.255.255  (1)

Binary conversion Samples:

/22 (1022 hosts, plus 2)
11111111.11111111.11111100.00000000
255 .     255    .   128 + 64 + 32 + 16 + 8 + 4 = 252 .   0

/23  (510 plus 2)
11111111.1111111.111111110.00000000
255      . 255   . 128 + 64+32+16+8+4+2 = 254  . 0

/27 (30 hosts per, plus 2)
11111111.11111111.11111111.11100000
255.255.255 .   128 + 64+32 = 224

/28 (14 hosts per, plus 2)
11111111.11111111.11111111.11110000
255.255.255.  128+64+32+16 = 240

/23 break into /28
28 - 23 = 5, 2 to the 5 (2x2x2x2x2= 32)