The Subnetting Formula Every Network Engineer Needs

Subnetting:
What is Subnetting?
Dividing a Large network into Multiple small networks is called subnetting.

What is CIDR?
CIDR stands for Classless inter domain routing,It is the total number of network bits.

Class A -- /8 ( N H H H)
Class B -- /16 (N N H H)
Class C -- /24 (N N N H)

Note : CIDR number is based on the how many Netwok portions are in Classes


• Converting Host bits into Network bits(i.e.converting O's into l's)

• Subnetting can be done based on requirement
Number of Networks Required'?
Number of Hosts Required?
Cisco Slash Notation

Note:-lt is very useful for Internet Service Providers(ISP),Large Organizations/ Companies etc.,

How the 255 is coming:
000-1
001-2
010-4
011-8
100-16
101-32
110-64
111-128
--------------
255

Requirement of Networks:
• A corporate network has 200PC's
• Which class of IPAddress is preferred for the network ?
Answer : Class C

• There are 4 department swith 50 pc's each

Marketing -+ 192.168.1.1 to 192.168.1.50
Sales -+ 192.168.1.51 to 192.168.1.100
Finance -+ 192.168.1.101 to 192.168.1.150
IT -+ 192.168.1.151 to 192.168.1.200

Administrators Requirement:
• Inter-department communication should not be there Solution:
• Allocate different Networks to each Department
i.e.,
Marketing -+ 192.168.1.1 to 192.168.1.50
Sales -+ 192.168.2.1 to 192.168.2.50
Finance -+ 192.168.3.1 to 192.168.3.50
IT -+ 192.168.4.1 to 192.168.4.50

Main Aim of Subnetting:
• Problem with the previous scenariois
Wastage of IP addresses,if it is Public IPaddresses(Approx.800)
To reduce the wastage of IP addresses,we have Subnetting

Powertable:
Requirement of Subnets-4no's?
• ClassC: 192.168.1.0
• Octet Format is N.N.N.H
Network bits:24 Host bits:8

• Subnets required:4no's
= 2n Req.ofSubnet
= 2" 4
= 22 4
= 4subnets

• No.of Hosts/Subnet
= 2no of hostbits-2
= 26-2(-2 is for NetworkID & BroadcastID)
= 64-2
= 62Hosts/Subnet
--= -

Continued...

Customized subnet mask

Requirement of Subnets-30no's?:
• ClassC:192.168.1.0
• Octet Format is N.N.N.H
Network bits:24 Host bits:8

• Subnets required:32no's
= 2° Req.ofSubnet
= 2" 4
= 25.!:4
= 32 subnets
• No.of Hosts/Subnet
= 2no of host bits-2
= 23-2(-2 is for NetworkID & BroadcastID)
= 8-2
= 6Hosts/Subnet

Requirement of Host-12no's?:
• ClassC:192.168.1.0
• Octet Format is N.N.N.H
Network bits:24 Host bits:8
• Host required:12no's
= 2"-2 Req.ofHost(-2 is for NetworkID & BroadcastID)
= 24-212
= 16-2
= 14Hosts
• No. of Subnets
= 2no of net workbits
= 24
= 16 subnets

Requirement of Host-2no1s?:
• ClassC:192.168.1.0
• Octet Format is N.N.N.H
Network bits:24 Host bits:8
• Host required :2no's
= 2"-2 Req.ofHost(-2is for Network ID & Broadcast ID)
= 22-2 2
= 4-2
= 2Hosts
• No. of Subnets
= 2no of network bits
= 26
=64subnets

Cisco Slash Notation-example-1
• ClassC:192.168.1.65/25

Network bits:25
• No.ofSubnets
= 2no of network bits
= 21
= 2subnets
• No.of Hosts/Subnet
=2no of hostbits-2

Hostbits:7

= 27-2(-2is for Network ID & Broadcast ID)
= 128-2
= 126Hosts/Subnet

Cisco Slash Notation-example-2:

• ClassC:192.168.1.65/27

Networkbits: 27
• No.of Subnets
= 2no of network bits
= 23
= 8subnets
• No.of Hosts/Subnet
= 2no of host bits-2

Host bits:5

= 25-2(-2 is for Network ID & Broadcast ID)
= 32-2
= 30Hosts/Subnet
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