Objectives  Sub netting
 CIDR
 Subnet Mask
 Formula for sub netting
 Class C Sub netting
 Class B Sub netting
 Class A Sub netting
With the rapid growth of the internet & the
ever-increasing demand for new IP
addresses, the standard address class
structure has been expanded by borrowing
bits from the Host portion to allow for more
Networks.



•
•
•
Dividing a single network into multiple
networks.
Converting Host bits into network bits. i.e.
converting 0’s into 1’s.
Sub netting can be done in three ways.
Requirement of Network
Requirement of Host
/ Notation



The key concept of sub netting is borrowing
bits from the host portion of the network to
create a sub network.
Rules govern this borrowing, ensuring that
some bits are left for a Host ID.
The rules require that two bits remain available
to use for the Host ID.
Bits available for Creating Subnets
Network bits
Host bits
Bits available
for subnet
CLASS A
8
24
24-2 = 22
CLASS B
16
16
16-2 = 14
CLASS C
24
8
8-2 = 6
Address class
Binary place
value
Binary
Digit
Sub Net mask value
128
64
32
16
8
4
2
1
1
0
0
0
0
0
0
0
128
1
1
0
0
0
0
0
0
192
1
1
1
0
0
0
0
0
224
1
1
1
1
0
0
0
0
240
1
1
1
1
1
0
0
0
248
1
1
1
1
1
1
0
0
252
1
1
1
1
1
1
1
0
254
1
1
1
1
1
1
1
1
255
Decimal
Equivalent
CIDR – Classless Inter-Domain Routing.
CIDR addressing method allows the use of a prefix to
designate the number of network bits in the mask
For Example: 200.15.1.53/25 (CIDR Notation)
In this example the CIDR value is /25 means the first 25
bits in the mask are network bits (1s)
Subnet mask : It is used to determine the network portion and
Host portion of an IP address.
Address Class
Subnet mask
Octet Format
CLASS A
255.0.0.0
N.H.H.H
CLASS B
255.255.0.0
N.N.H.H
CLASS C
255.255.255.0
N.N.N.H
N = Network
H = Host
Network
Host
Valid Host
=
=
=
2n [n = number of ON bits (1’s)]
2h [h = number of OFF bits (0’s)]
Host – 2 *
 * First IP (Host) of Network is use for network address
 * Last IP (Host) of network is use for Broadcast address
IP Address
Subnet Mask
192.168.1.0
255.255.255.0
Binary format
11111111.11111111.11111111.00000000
Network Bits
Host Bits
 192.168.1.0 divided into in 64 Hosts.
Subnet mask
: 255.255.255.0
Binary form
CIDR
: 11111111.11111111.11111111.00000000
: /24
Network
: 2n
=> 22
=4
Host
: 2h
=> 26
= 64
Valid Host
Block size
: Host – 2 => 64 – 2 = 62
: 256 – 192 = 64
New Binary form : 11111111.11111111.11111111.11000000
New Subnet mask : 255.255.255.192
New CIDR
: /26
n = no. of
ON bits.
h = no. of
OFF bits
 192.168.1.0 divided into in 64 Hosts.
No
.
Network/Network
ID
Broadcast ID
First Host
Last Host
1
192.168.1.0
192.168.1.63
192.168.1.1
192.168.1.62
2
192.168.1.64
192.168.1.127
192.168.1.65
192.168.1.126
3
192.168.1.128
192.168.1.191
192.168.1.129
192.168.1.190
4.
192168.1.192
192.168.1.255
192.168.1.193
192.168.1.254
 172.18.0.0 divided into in 8 Networks.
Subnet mask
: 255.255.0.0
Binary form
CIDR
: 11111111.11111111.00000000.00000000
: /16
Network
: 2n
=> 23
Host
: 2h
=> 213
Valid Host
: Host – 2 => 8192 – 2 = 8190
Block Size
: 256 – 224 = 32
=8
= 8192
New Binary form : 11111111.11111111.11100000.00000000
New Subnet mask : 255.255.224.0
New CIDR
: /19
n = no. of
ON bits.
h = no. of
OFF bits
 172.18.0.0 divided into in 8 Networks.
S No
Network/Network ID
Broadcast ID
First Host
Last Host
1
172.18.0.0
172.18.31.255
172.18.0.1
172.18.31.254
2
172.18.32.0
172.18.63.255
172.18.32.1
172.18.63.254
3
172.18.64.0
172.18.91.255
172.18.64.1
172.18.91.254
4
172.18.92.0
172.18.127.255
172.18.92.1
172.18.127.254
5
172.18.128.0
172.18.159.255
172.18.128.1
172.18.159.254
6
172.18.160.0
172.18.191.255
172.18.160.1
172.18.191.254
7
172.18.192.0
172.18.223.255
172.18.192.1
172.18.223.254
8
172.18.224.0
172.18.255.255
172.18.224.1
172.18.255.254
 Find the Broadcast ID of this network 110.60.21.15/11
Subnet mask
: 255.0.0.0
Binary form
CIDR
: 11111111.00000000.00000000.00000000
: /8
Network
: 2n
=> 23
Host
: 2h
=> 221
Valid Host
: Host – 2 => 2097152 – 2 = 2097150
Block Size
: 256 – 224 = 32
=8
= 2097152
New Binary form : 11111111.11100000.00000000.00000000
New Subnet mask : 255.224.0.0
New CIDR
: /11
n = no. of
ON bits.
h = no. of
OFF bits
 Find the Broadcast ID of this address 110.60.21.15/11
S No
Network/Network ID
Broadcast ID
First Host
Last Host
1
110.0.0.0
110.31.255.255
110.0.0.1
110.31.255.254
22
110.32.0.0
110.32.0.0
110.63.255.255
110.32.0.1
110.63.255.254
3
110.64.0.0
110.91.255.255
110.64.0.1
110.91.255.254
4
110.92.0.0
110.127.255.255
110.92.0.1
110.127.255.254
5
110.128.0.0
110.159.255.255
110.128.0.1
110.159.255.254
6
110.160.0.0
110.191.255.255
110.160.0.1
110.191.255.254
7
110.192.0.0
110.223.255.255
110.192.0.1
110.223.255.254
8
110.224.0.0
110.255.255.255
110.224.0.1
110.255.255.254
Thanks