Introduction to Subnetworks

Marandola—Lets have some FUN ????

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

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

How Do You Create Subnets?• Bits are Borrowed from the Host Field

– This Creates a Subnet Field in the IP address

Class C SubnetsNetworkNetwork Network Host

S HH H H H HS

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 networkHow many bits can be borrowed from a Class B network? From a Class A network?

Class C SubnetsNetworkNetwork Network Host

S HH H H H HS

The number of Subnets Created is calculated using the following formula:

# Subnets Created = 2# Borrowed Bits

Class C SubnetsS HH H H H HS

Borrow 2 Bits = 22 = 4 Subnets

S SS H H H HS

Borrow 4 Bits = 24 = 16 Subnets

S HS H H H HS

Borrow 3 Bits = 23 = 8 Subnets

Class C SubnetsS SS S H H HS

Borrow 5 Bits = 25 = 32 Subnets

Borrow 7 Bits = CannotTwo Host Bits Must Remain

S SS S S H HS

Borrow 6 Bits = 26 = 64 Subnets

How Many Subnets?Borrowed

Class Bits #SubnetsA,B,C 2 4 A,B,C 3 8 A,B,C 4 16 A,B,C 5 32 A,B,C 6 64 A,B 7 128 A,B 8 256 A,B 9 512 A,B 10 1,024 A,B 11 2,048 A,B 12 4,096 A,B 13 8,192 A,B 14 16,384 A 15 32,768 A 16 65,536 A 17 131,072 A 18 262,144 A 19 524,288 A 20 1,048,576 A 21 2,097,152 A 22 4,194,304

• 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!

How Many Subnets?Borrow 2 Bits = 22 = 4 Subnets

How Many Subnets?Borrowed Available

Class Bits #Subnets SubnetsA,B,C 2 4 2 A,B,C 3 8 6 A,B,C 4 16 14 A,B,C 5 32 30 A,B,C 6 64 62 A,B 7 128 126 A,B 8 256 254 A,B 9 512 510 A,B 10 1,024 1,022 A,B 11 2,048 2,046 A,B 12 4,096 4,094 A,B 13 8,192 8,190 A,B 14 16,384 16,382 A 15 32,768 32,766 A 16 65,536 65,534 A 17 131,072 131,070 A 18 262,144 262,142 A 19 524,288 524,286 A 20 1,048,576 ########A 21 2,097,152 ########

How Many Hosts/Subnet?NetworkNetwork Network Host

S HH H H H HS

The number of Hosts per subnet is calculated using the following formula:

# Hosts/Subnet = 2# Host Bits Remaining

# Hosts = 26 = 64 hosts/subnet

• 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.

How Many Hosts/Subnet?6 Host Bits Remain = 26 = 64 Hosts

How Many Hosts/Subnet?Borrowed Remaining Available

Class Bits Host Bits #Hosts HostsC 2 6 64 62 C 3 5 32 30 C 4 4 16 14 C 5 3 8 6 C 6 2 4 2 B 7 9 512 510 B 8 8 256 254 B 9 7 128 126 B 10 6 64 62 B 11 5 32 30 B 12 4 16 14 B 13 3 8 6 B 14 2 4 2

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

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

Formulas to Remember!

# Subnets Created = 2# Borrowed Bits

# Hosts/Subnet = 2# Host Bits Remaining

• 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!

Subnetting Made Easier!