Subnet Mask
CIDR
Net Bits
Host Bits
# of Nets
Hosts/Net
Total Hosts
255.255.0.0
/16
0
16
1
65534
65534
255.255.128.0
/17
1
15
0(2*)
32766
0(65534*)
255.255.192.0
/18
2
14
2(4*)
16382
32764(65528*)
255.255.224.0
/19
3
13
6(8*)
8190
49140(65520*)
255.255.240.0
/20
4
12
14(16*)
4094
57316(65504*)
255.255.248.0
/21
5
11
30(32*)
2046
61380(65472*)
255.255.252.0
/22
6
10
62(64*)
1022
63364(65408*)
255.255.254.0
/23
7
9
126(128*)
510
64260(65280*)
255.255.255.0
/24
8
8
254(256*)
254
64516(65024*)
255.255.255.128
/25
9
7
510(512*)
126
64260(64512*)
255.255.255.192
/26
10
6
1022(1024*)
62
63364(63488*)
255.255.255.224
/27
11
5
2046(2048*)
30
61380(61440*)
255.255.255.240
/28
12
4
4094(4096*)
14
57316(57344*)
255.255.255.248
/29
13
3
8190(8192*)
6
49140(49152*)
255.255.255.252
/30
14
2
16382(16384*)
2
32764(32768*)
255.255.255.254
/31
15
1
32766(32768*)
0*
0(32768**)
255.255.255.255
/32
16
0
65534(65536*)
1***
65534(65536*)
[*] According to classic IP routing rules you are not able to use the subnets with all zeros or ones in the network portion. However most modern machines have no trouble actually using the upper and lower subnets. Steven C. Jensen has a very good discussion of lower/upper [...]