D-Link DES-3326 User Manual page 44

This example uses an 11-bit subnet mask. (There are 3 additional bits added to the default Class A
subnet mask). So the number of subnets is:
3
2 – 2 = 8 – 2 = 6
Subnets of all "0"s and all "1"s are not allowed, so 2 subnets are subtracted from the total.
The number of bits used in the node part of the address is 24 – 3 = 21 bits, so the total number of
nodes is:
21
2 – 2 = 2,097,152 – 2 = 2,097,150
Multiplying the number of subnets times the number of nodes gives 12,582,900 possible nodes.
Note that this is less than the 16,777,214 possible nodes that an unsubnetted class A network would
have.
Subnetting reduces the number of possible nodes for a given network, but increases the segmentation
of the network.
Classless InterDomain Routing – CIDR
Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of specifying all of
the bits of the subnet mask, it is simply listed as the number of contiguous "1"s (bits) in the network
portion of the address.
11111111.11100000.00000000.00000000 – and you can see that there are 11 "1"s or 11 bits used to
mask the network address from the node address. Written in CIDR notation this becomes:
10.32.0.0/11
# of Subnet Mask
Bits
2 255.192.0.0
3 255.224.0.0
4 255.240.0.0
5 255.248.0.0
6 255.252.0.0
7 255.254.0.0
8 255.255.0.0
9 255.255.128.0
10 255.255.192.0
11 255.255.224.0
12 255.255.240.0
13 255.255.248.0
14 255.255.252.0
15 255.255.254.0
16 255.255.255.0
17 255.255.255.1
28
18 255.255.255.1
92
19 255.255.255.2
24
20 255.255.255.2
40
21 255.255.255.2
48
22 255.255.255.2
52
Look at the
CID # of # of Hosts
R Subnets
Nota
tion
/10 2 4194302
/11 6 2097150
/12 14 1048574
/13 30 524286
/14 62 262142
/15 126 131070
/16 254 65534
/17 510 32766
/18 1022 16382
/19 2046 8190
/20 4094 4094
/21 8190 2046
/22 16382 1022
/23 32766 510
/24 65534 254
/25 131070 126
/26 262142 62
/27 525286 30
/28 104857 14
4
/29 209715 6
0
/30 419430 2
2
Table 5-5. Class A Subnet Masks
DES-3326 Layer 3 Fast Ethernet Switch User's Guide
subnet mask of
44
the above example
Total Hosts
8388604
12582900
14680036
15728580
16252804
16514820
16645636
16710660
16742404
16756740
16760836
16756740
16742404
16710660
16645636
16514820
16252804
15728580
14680036
12582900
8388604
in binary -