D-Link DES-3250TG User Manual page 33

Some restrictions apply to subnet addresses. Addresses of all "0"s and all "1"s are reserved for the local
network (when a host does not know it's network address) and for all hosts on the network (the
broadcast address). This also applies to subnets. A subnet address cannot be all "0"s or all "1"s. A 1-bit
subnet mask is also not allowed.
Calculating the Number of Subnets and Nodes
To calculate the number of subnets and nodes, use the formula (2
either the subnet mask or the node portion of the IP address. Multiplying the number of subnets by the
number of nodes available per subnet gives the total number of nodes for the entire network.
Example
00001010.00101010.01001001.11010010
11111111.11100000.00000000.00000000
00001010.00100000.00000000.00000000
00001010.00101010.11111111.11111111
This example uses an 11-bit subnet mask. (There are three 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 two 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 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
D-Link DES-3250TG Standalone Layer 2 Switch
10.42.73.210
255.224.0.0
10.32.0.0
10.32.255.255
Look at the
# 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
Class A IP address
Subnet Mask
Network Address
Broadcast Address
subnet mask of
CID # of # of
R Subnets Hosts
Nota
tion
/10 2 419430
2
/11 6 209715
0
/12 14 104857
4
/13 30 524286
/14 62 262142
/15 126 131070
/16 254 65534
25
– 2) where n = the number of bits in
n
the above example
Total Hosts
8388604
12582900
14680036
15728580
16252804
16514820
16645636
in binary -