Introduction%20to%20Subnetworks

1
Introduction to Subnetworks

• MarandolaLets have some FUN ????

2
What Is a Subnet?

• A Series of Networks within a Network
• Created by subdividing the Host address field and
  creating a Subnetwork Field
• All Hosts on a Subnetwork share a common
  subnetwork address

3
Why Subnet a Network?

• Provides Greater Organization of Large Networks
  (Class A 16 Million Hosts!)
• Allows Additional Networks (subnets) without
  applying for additional IPs
• Gives local administrators more control
• Provides a Third Level of Hierarchy
• Reduces the Size of Broadcast Domains

4
How Do You Create Subnets?

• Bits are Borrowed from the Host Field
• This Creates a Subnet Field in the IP address

5
Class C Subnets
Network
Network
Network
Host
S
H
H
H
H
H
H
S
Two Bits Borrowed from the Host Field to form a
third layer of hierarchy - A Subnet Field
Two Bits must always remain so a maximum of 6
Bits may be borrowed from a Class C network
How many bits can be borrowed from a Class B
network? From a Class A network?
6
Class C Subnets
Network
Network
Network
Host
S
H
H
H
H
H
H
S
The number of Subnets Created is calculated using
the following formula
Subnets Created 2 Borrowed Bits
7
Class C Subnets
S
H
H
H
H
H
H
S
Borrow 2 Bits 22 4 Subnets
S
H
S
H
H
H
H
S
Borrow 3 Bits 23 8 Subnets
S
S
S
H
H
H
H
S
Borrow 4 Bits 24 16 Subnets
8
Class C Subnets
S
S
S
S
H
H
H
S
Borrow 5 Bits 25 32 Subnets
S
S
S
S
S
H
H
S
Borrow 6 Bits 26 64 Subnets
Borrow 7 Bits Cannot Two Host Bits Must Remain
9
How Many Subnets?
10
How Many Subnets?
Borrow 2 Bits 22 4 Subnets

• If you Borrow 2 Host Bits you do NOT get 4
  Subnets. Why?
• Remember the Network Address and Broadcast
  Address - Both of these addresses are Reserved,
  they cannot be used!

11
How Many Subnets?
12
How Many Hosts/Subnet?
Network
Network
Network
Host
S
H
H
H
H
H
H
S
The number of Hosts per subnet is calculated
using the following formula
Hosts/Subnet 2 Host Bits Remaining
Hosts 26 64 hosts/subnet
13
How Many Hosts/Subnet?
6 Host Bits Remain 26 64 Hosts

• If there are 6 Host Bits remaining you do NOT
  get 64 Hosts/Subnet. Why?
• Each subnetwork has its own Subnetwork Address
  and Broadcast Address - Both of these addresses
  are Reserved and cannot be used!
• Thus only 62 Hosts are available.

14
How Many Hosts/Subnet?
15
Formulas to Remember!
Subnets Created 2 Borrowed Bits

• Remember to subtract 2 for the Network Address
  and Broadcast Address.
• Remember to subtract 2 for the Subnetwork Address
  and Subnetwork Broadcast Address.

Hosts/Subnet 2 Host Bits Remaining
16
Subnetting Made Easier!

• Subnetting is not an easy concept to grasp but
  you can make life easier on yourself if you will
  Remember the formulas and rules.
• Failure to remember this information will
  guarantee your inability to subnet a network.
• If you require assistance with Subnetting I will
  be happy to help. Flash cards work great!