This example uses an
23 – 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:
221 – 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. Look at the subnet mask of the above example in binary - 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 | CID | # of | # of Hosts | Total Hosts |
Bits |
| R | Subnets |
|
|
|
| Nota |
|
|
|
|
| tion |
|
|
|
2 | 255.192.0.0 | /10 | 2 | 4194302 | 8388604 |
3 | 255.224.0.0 | /11 | 6 | 2097150 | 12582900 |
4 | 255.240.0.0 | /12 | 14 | 1048574 | 14680036 |
5 | 255.248.0.0 | /13 | 30 | 524286 | 15728580 |
6 | 255.252.0.0 | /14 | 62 | 262142 | 16252804 |
7 | 255.254.0.0 | /15 | 126 | 131070 | 16514820 |
8 | 255.255.0.0 | /16 | 254 | 65534 | 16645636 |
9 | 255.255.128.0 | /17 | 510 | 32766 | 16710660 |
10 | 255.255.192.0 | /18 | 1022 | 16382 | 16742404 |
11 | 255.255.224.0 | /19 | 2046 | 8190 | 16756740 |
12 | 255.255.240.0 | /20 | 4094 | 4094 | 16760836 |
13 | 255.255.248.0 | /21 | 8190 | 2046 | 16756740 |
14 | 255.255.252.0 | /22 | 16382 | 1022 | 16742404 |
15 | 255.255.254.0 | /23 | 32766 | 510 | 16710660 |
16 | 255.255.255.0 | /24 | 65534 | 254 | 16645636 |
17 | 255.255.255.1 | /25 | 131070 | 126 | 16514820 |
| 28 |
|
|
|
|
18 | 255.255.255.1 | /26 | 262142 | 62 | 16252804 |
| 92 |
|
|
|
|
19 | 255.255.255.2 | /27 | 525286 | 30 | 15728580 |
| 24 |
|
|
|
|
20 | 255.255.255.2 | /28 | 104857 | 14 | 14680036 |
| 40 |
| 4 |
|
|
21 | 255.255.255.2 | /29 | 209715 | 6 | 12582900 |
| 48 |
| 0 |
|
|
22 | 255.255.255.2 | /30 | 419430 | 2 | 8388604 |
| 52 |
| 2 |
|
|
Table 5-5. Class A Subnet Masks
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