DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
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:
2
3
– 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:
2
21
– 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
Bits
Subnet Mask CID
R
Nota
tion
# of
Subnets
# of Hosts Total Hosts
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
28
/25 131070 126 16514820
18 255.255.255.1
92
/26 262142 62 16252804
19 255.255.255.2
24
/27 525286 30 15728580
20 255.255.255.2
40
/28 104857
4
14 14680036
21 255.255.255.2
48
/29 209715
0
6 12582900
22 255.255.255.2
52
/30 419430
2
2 8388604
Table 5-5. Class A Subnet Masks
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