Subnetting
IP Addresses

IP addresses identify hosts and network devices
 To send and receive messages on an IP network, every
host must be assigned a unique 32-bit IP address
 IP address are displayed in dotted-decimal notation

192.168.1.1

Each of the 4 octets represents 8 bits

IP addresses are hierarchical


The network portion identifies the network that a host
belongs to
The host portion identifies an individual host on a
network
Network Addresses

The network portion of the address, is used to
represent the entire network


It represents a group of IP addresses that can be used on
that network
The network address consists of the network field
plus all 0’s in the host portion of the address


192.168.18.00000000
192.168.18.0
The Network address is not a usable host IP address
 Network addresses are only used by routers to decide
how to get packets to their destination

Host vs. Network Portion
Network Number
Host Number
Broadcast Address

A Broadcast Address is the address used to
send messages to every host on the same
network
 A Broadcast Address consists of the Network
address, plus all 1’s in the host field
 The Broadcast address is NOT a USABLE host
address and can not be assigned to a host
Broadcast Addresses
Network Address
 120.0.0.0
 170.50.0.0.
 192.168.10
Broadcast Address
120.255.255.255
170.5.255.255
192.168.10.255
Usable Host Addresses


As we just saw, the Network address and the
Broadcast address are NOT usable host addresses
A usable host IP address is an IP address that:





Is not a Network Address (all 0’s in host field)
Is not a Broadcast Address (all 1’s in host field)
Is not a reserved Address (127 addresses)
Is a Class A, B or C address
Only a usable host IP address can be assigned to a
host device
Determining Usable Host
Addresses
Network
10.0.0.0
172.16.0.0
192.168.1.0
Usable Hosts
10.0.0.1 – 10.255.255.254
172.16.0.1-172.16.255.254
192.168.1.1-192.168.1.254
Broadcast
10.255.255.255
172.16.255.255
192.168.1.255
Available Host Addresses

The number of available host addresses on a network can be
calculated with the formula:
2 ^ host bits – 2
Network type
255.0.0.0
2 ^ 24 -2 =
16, 277, 214
 255.255.0.0
2 ^ 16- 2 =
65, 534
 255.255.255.0
2^8–2=
254
The reason we always subtract 2 from the total host addresses to
determine the available host addresses, is because the network
address and broadcast address are NOT usable host address
Therefore, every network has 2 addresses that can not be
assigned to hosts, the very 1st address (all 0’s in the host portion)
and the very last address (all 1’s in the host portion)



Available Hosts
IP Address Classes

To create more possible network designations, the 32-bit
address space was organized into five classes.
 Class A, B, and C: Commercial networks
 Class D and E: multicast and experimental

The class of a network is indicated by the values of the first few bits of
the IP address, called the high-order bits.
Commercial IP Address Classes

Class C addresses are usually assigned to small networks





N.N.N.H
Class B addresses are typically used for medium-sized networks





Use 3 octets for the network and 1 for the hosts
The default subnet mask is 24 bits: 255.255.255.0
2, 097, 150 (2 ^ 21 – 2) possible networks
254 (2 ^ 8 – 2) available host addresses per network
Use 2 octets for the network and 2 for the hosts
N.N.H.H
The default subnet mask is 16 bits: 255.255.0.0
16, 382 (2 ^ 14 – 2) possible networks
65, 534 (2 ^ 16 – 2) available host addresses per network
Class A addresses are typically assigned to large organizations.




Use 1 octet for the network and 3 for the hosts N.H.H.H
The default subnet mask is 8 bits: 255.0.0.0
126 (2 ^ 7 – 2) possible networks
16, 777, 214 (2 ^ 24 – 2) available host addresses per network
Class A

The first bit is always 0
 Addresses start with 0 to 126
Class B

First two bits are always 1 and 0
 Addresses start with 128 to 191
Class C

First three bits are always 1, 1 and 0
 Addresses start with 192 to 223
The value of the 1st Octet

The class of an address can be determined by the value
of the first octet
 Each Class of IP Address can only begin with a certain
value
 Also notice, that there is a leading bit pattern that never
changes for each class of addresses
Class
Class A
 Class B
 Class C
 Class D
 Class E

Decimal Value of 1st Octet
1 - 126
128 – 191
192 – 223
224 – 239
240 – 255
Leading Bit value
0
10
110
1110
1111
1 to 126
Private Network Addresses
Private Network
Addresses
The Loopback Address
 There
are also private addresses that can be
used for the diagnostic testing of devices.
 This type of private address is known as a
loopback address.
 The class A, 127.0.0.0 network address, is
reserved for loopback testing.
 The loopback IP address, 127.0.0.1 is used to
test a NIC card to verify that it is sending and
receiving signals.
Subnet Masks

A subnet mask is a 32 bit address which tells devices
which part of the IP address is network and which part is
host

Let routers & hosts figure out which network or subnet an IP
address belongs to
 Subnet
Masks contain:
all 1’s in the network field
 all 0’s in the host field


Example Subnet Masks:




255.255.255.0
255.255.0.0
255.255.255.128
255.254.0.0
Subnet Mask Formats
Subnet Masks can be written in 2 different formats:
1.
Dotted Decimal format

2.
192.168.1.1 255.255.255.0
Bit-Mask Format

192.168.1.1 /24

This indicates that there are 24 bits ( 24 1’s) in the
network and subnetwork portion of the address
(255.255.255.0)
Default Subnet Masks


A default Subnet Mask is used when a network has NOT
been subnetted
Default Subnet Masks
 Class




/8
B: 255.255.0.0
OR
/16
11111111. 11111111.00000000.00000000
16 bits for network, 16 bits for host
 Class

OR
11111111.00000000.00000000.00000000
8 bits for network, 24 bits for host
 Class

A: 255.0.0.0
C: 255.255.255.0 OR
11111111.11111111.1111111.00000000
24 bits for network, 8 bits for host
/24
Subnettting
Subnetting…



Is dividing a network address into smaller divisions, or
subnets
It allows you to divide a Class A, B or C network address
into smaller subnetworks, with their own network
addresses
Each subnet acts like its own LAN and is connected to
its own router interface
Class C Network: 192.168.1.0

Addresses Available:



Goal:


Need 4 separate network addresses for 4 company LANs
Solution:


192.168.1.0 - 192.168.1.255 (256 total)
192.168.1.1 - 192.168.1.254 are usable (254 usable)
Subnet the Network Address
How?


256 Addresses to work with
256 / 4 Subnets needed = 64 Addresses per Subnet
Subnet 1
0 to 63
Subnet 2
64 to 127
Subnet 3
Subnet 4
129 to 191 192 to 255
Sample Subnetting Scheme
Network Address Assigned:
Subnets Created:
Why Subnet a Network?
1.
Provides better organization of Large Networks


2.
Gives local administrators more control over their
local networks

3.
more efficient assignment of addresses
breaks a network into smaller sections, which act like
separate networks, without applying for additional Network
addresses
adds security between networks
Reduces the size of the broadcast domain

Helps minimize traffic loads by restricting broadcast traffic to
smaller area
Classful Subnetting

Routers see each network as a Single network with a
single network number.




This keeps routing tables small
Each network is identified to routers by its network number
Ex. Given a local IP address of 192.168.10.14, the world outside
your LAN sees only the advertised major network number of
192.168.10.0.
Only internal routers (within your organization) use the
Subnet Addresses to route between your local
networks

These routers use the subnet addresses to recognize different
subnets within your whole network – all other routers ignore it!
Subnet Address

A subnet address is the address that
identifies a subnet (similar to a network
address)
 It contains:




All 1’s in the network and Subnet Fields
All 0’s in the Host fields
Subnet addresses are created by borrowing bits from the Host
field of a network address and making them part of the network
Field
All Hosts on a Subnetwork share a common subnetwork
address- they are all on the same network or subnet
Creating Subnets

To create Subnets:


Bits are Borrowed from the Host Field and made
part of the network field
This Creates a Subnet Field in the IP address
Network
Network
S
Network
S
H
H
Host
H
H
H
H
•2 Bits are borrowed from the Host Field to make a Subnet
Field, which becomes part of the Subnet Address
Custom Subnet Masks




When a network has been subnetted, it no longer uses
the Default Subnet Mask
A subnetted network must use a Custom Subnet Mask,
which tells the router how many bits have been
borrowed from the host portion of the address
The router must use a modified or custom subnet mask
to distinguish the subnets from each other
The Custom Subnet Mask is created by
borrowing bits from the host portion of the
default subnet mask, and changing them from
0’s to 1’s
Create a Custom Subnet Mask

Example: A Class A subnet mask, with 2 bits borrowed:
 Default Mask = 255.0.0.0
 2 bits borrowed = 255.11000000.0.0
 Custom Subnet Mask = 255.192.0.0
 Example: A class B subnet mask with 3 bits borrowed
 Default Mask = 255.255.0.0
 2 bits borrowed = 255.255.11100000.0
 Custom Subnet Mask = 255.255.224.0
 Example: A class C subnet mask with 4 bits borrowed:
 Default Mask = 255.255.255.0
 2 bits borrowed = 255.255.255.1111
 Custom Subnet Mask = 255.255.255.240
Subnet Mask Bit Patterns
Recognizing Bit Patterns:
10000000 = 128
11000000 = 192
11100000 = 224
11110000 = 240
11111000 = 248
11111100 = 252
11111110 = 254
11111111 = 255
How many bits to borrow?

So how do you know how many bits to borrow to
Subnet and to create the Custom Subnet Mask?
 How many bits you borrow depends on How
MANY SUBNETS you need to create!

Use these Formulas:

Total subnets created = 2 x


(x = # of bits borrowed)
Usable subnets created = 2 x -2

(2 less than the total subnets)
Subnetting Rules

A Minimum of 2 bits must be borrowed
 A Minimum of 2 bits must be left over
 You can’t use the 1st subnet created because
it contains the network address for the entire
network
 You can’t use the last subnet created because
it contains the broadcast address for the
entire network
Number of Subnets Created
S
S
H
H
H
H
H
H
2 Bits Borrowed= 22 = 4 Subnets, 2 usable
S
S
S
H
H
H
H
H
3 Bits Borrowed = 23 = 8 Subnets, 6 usable
S
S
S
S
H
H
H
H
4 Bits Borrowed = 24 = 16 Subnets, 14 usable
Number of Subnets Created
S
S
S
S
S
H
H
H
5 Bits Borrowed = 25 = 32 Subnets, 30 usable
S
S
S
S
S
S
H
H
6 Bits Borrowed = 26 = 64 Subnets, 62 usable
How Many Host Addresses per
Subnet?



Often it is very important to know not only how MANY
SUBNETS will be created, but also how MANY HOST
ADDRESSES will be available to each subnet
How many host addresses will be available on each
subnet is determine by how many bits are left in the host
field of the address
Use these formulas to determine how many host
addresses are available to each subnet:
 Total host addresses per subnet = 2 y


y = the number of bits left over in the host portion of the
address (bits not borrowed)
Usable host addresses per subnet = 2 y -2

2 less than the total
How Many Host Addresses per Subnet?
Network
S
Network
S
H
Network
H
H
Host
H
H
H
6 Bits are left in the Host field of the address
2 Bits borrowed = 2 usable Subnets
Host Addresses = 26 (64) per subnet
Usable Addresses = 62 per subnet
What about the other 2 Addresses?






Why is the usable number of host addresses 2 less than the total
number of addresses available?
Each subnetwork has its own Subnetwork Address and
Broadcast Address
Both of these addresses are Reserved and cannot be used!
The 1st IP address in each subnet becomes the Subnetwork
Address
The Last IP address in each subnet becomes the Broadcast
Address
Thus 2 addresses in each subnet cannot be used as host
addresses
The Trade-Off
An unfortunate by-product of creating subnetworks is that
the reserved network and broadcast numbers can not be
used in each subnetwork. Every Subnet created creates 2
more unusable addresses in the network.
Formulas to Remember!
# Total Subnets Created = 2# Bits Borrowed
# Usable Subnets Created = 2# Bits Borrowed - 2
# Total Hosts/Subnet = 2# Host Bits Remaining
# Usable Hosts/Subnet = 2# Host Bits Remaining - 2
Custom Subnet Mask Problem
The network 192.56.32.0 needs to be divided into 2
usable subnets, what is the custom subnet mask?
• What class is the address? C
• What is the default subnet mask? 255.255.255.0
• We need 2 usable subnets – how many bits do we need to
borrow? 2
• What is the Custom subnet mask? 255.255.255.192
• 8 bits are in the host portion of address (00000000)
• We need to borrow 2 bits from the left side of the host portion
of the default subnet mask to create our subnet mask
NNNNNNNN.NNNNNNNN.NNNNNNNN.SShhhhhh
11111111. 11111111.
11111111. 11000000
Custom Subnet Mask Problems
The network 177.56.0.0 needs 3 usable
subnetworks, what would it’s custom
subnet mask be?





What class is this address B
What is the default subnet mask? 255.255.0.0
How many bits need to be borrowed? 3 (8 total, 6 usable)
What is the custom subnet mask?
255.255.224.0
11111111.11111111.11100000.00000000
How many bits have been borrowed to for the
address 205.10.7.16, with mask 255.255.255.240?
How many usable subnets are there?

What class is this address? C

What is the default subnet mask? 255.255.255.0

How many bits were borrowed to make this mask? 4

How many usable subnets will that create? 14
Subnetting

When we subnet a network, these are the things that
must be determined:
1.
2.
Custom Subnet Mask
Subnet Address for each Subnet



3.
Broadcast Address for each Subnet



4.
The address used to send messages to all hosts in the subnet
The last address in each subnet range
Contains all 1’s in the host portion of the Subnet Address
Usable Host addresses for each subnet


The address that identifies the entire subnet – only used by
routers (like a network address)
Contain’s all 1’s in the network/subnet portion of the address and
all 0’s in the host portion of the address
The 1st address in each Subnet range
All the addresses in each subnet that can be assigned to hosts
We now know how to create the Custom Subnet Mask,
but how do we determine the Subnet Information?
Subnetting

These are the steps we use to Subnet:
1.
2.
3.
4.
5.
6.
7.
Determine what class the Network address is
Determine the Default Subnet Mask
Determine How many bits need to be borrowed to
create the needed # of subnets
Create the Custom Subnet Mask
Determine the Subnet Addresses
Determine the Broadcast Address for each subnet
Determine the usable host addresses for each
subnet
Subnetting Problem
The network 192.168.10.0 needs 6 usable subnets



1.
2.
3.
Find the subnetwork address for each subnet
Find the ranges usable host IP addresses for each subnet
Find the broadcast address for each subnet
What is the Class? C
What is the Default Subnet Mask? 255.255.255.0
How many bits need to be borrowed?
•Need 6 usable subnets.
•3 bits = 6 usable subnets
4.
What is the custom Subnet Mask?
11111111.11111111.11111111.11100000 = 255.255.255. 224
Subnetting Problem
5. Determine the Subnet Addresses
•
Use binary counting to begin converting the 0’s in the subnet portion to 1’s
(starting from the last bit borrowed)
192.168.10.SSSHHHHH
192.168.10.000|00000 – 192.168.10.0 192.168.10.001|00000 – 192.168.10.32 –
192.168.10.010|00000 –192.168.10.64 –
192.168.10.011|00000 – 192.168.10.96 –
192.168.10.100|00000 – 192.168.10.128 –
192.168.10.101|00000 – 192.168.10.160–
192.168.10.110|00000 – 192.168.10.192–
192.168.10.111|00000 – 192.168.10.224 –
1st
2nd
3rd
4th
5th
6th
7th
8th
(not usable)
(1st usable)
(2nd usable)
(3rd usable)
(4th usable)
(5th usable)
(6th usable)
(not usable)
Subnetting Problem
6.
7.
Determine the Broadcast Address for each subnet
Determine the usable host addresses for each
subnet
Usable Subnet
Subnet Address
Usable Hosts
Broadcast Address
Class B Subnetting
The network 177.56.45.13 needs 7 usable subnetworks –
What is the Subnetwork Address, Broadcast
Address and Host Range for the 3rd subnet
What class is this address? B
2. What is the default subnet mask? 255.255.0.0
3. How many bits need to be borrowed to get 4
4 bits
usable subnets?
1.
23= 6 usable subnets
4.
What is the custom subnet mask?
255.255.11110000.0
255.255.240.0
5. Determine Subnet Addresses
Original Network: 177.56.hhhhhhhh.hhhhhhhh
Subnetted Network: 177.56.SSSShhhh.hhhhhhhh
177.56.0000|0000.0 – 1st subnet, not usable
177.56.0001|0000.0– 177.56.16.0 – 2nd subnet, 1st usable
177.56.0010|0000.0 – 177.56.32.0 – 3rd subnet, 2nd usable
177.56.0011|0000.0 – 177.96.48.0 – 4th subnet, 3rd usable
177.56.0100|0000.0 – 177.56.64.0 – 5th subnet, 4th usable
177.56.0101|0000.0 – 177.56.80.0 – 6th subnet, 5th usable
177.56.0110|0000.0 – 177.56.96.0 – 7th subnet, 6th usable
177.56.0111|0000.0 – 177.56.112.0 – 8th subnet, 7th usable
=========================================
177.56.1111|0000.0 – 177.56.240.0- last subnet, not usable
NOTE: Do you see a pattern above? The place value of the last subnetted bit
is also the increment by which the subnetwork numbers will increase!!!!
Subnetting Shortcut

The value of the Subnet addresses increases by
the value of the LAST BIT BORROWED
The place value of the last subnetted bit is also the increment
by which the subnetwork numbers will increase!!!!
Ex. Subnet Mask 255.255.255.240
Borrow 4 bits: SSSS|hhhh
•The place value of the last bit borrowed = 16
•Subnetwork numbers = .0, .16, .32, .48, .64, .80, etc.
Subnetting Shortcuts
If you borrow
For Class A, B and C
2 bits
11000000
3 bits
11100000
4 bits
11110000
5 bits
11111000
6 bits
11111100
For Class A and B only
7 bits
11111110
8 bits
11111111
Subnets increase by
Ex:
64
32
16
8
4
192.168.1.64
172.16.32.0
10.16.0.0
192.168.1.8
120.4.0.0
2
1
150.1.2.0
90.1.0.0
Class B Subnetting
Determine the Broadcast Address for the
firstst 6 subnets
7. Determine the usable host addresses the
first 6 subnets
6.
Subnet
Subnet
Address
Host Addresses
Broadcast
Address
1st
177.56.16.0
177.56.16.1 - 177.56.31.254
177.56.31.255
2nd
177.56.32.0
177.56.32.1 – 177.56.47.254
177.56.47.255
3rd
177.56.48.0
177.56.48.1 -177.56.63.254
177.56.63.255
4th
177.56.64.0
177.56.64.1 -177.56.79.254
177.56.79.255
5th
177.56.80.0
177.56.80.1-177.56.95.254
177.56.95.255
6th
177.56.96.0
177.56.96.1- 177.56.111.254
177.56.111.255
Find info for the 2nd usable subnet

Subnetwork Address:


Broadcast Address:


177.56.32.0
177.56.47.255
NOTE: The broadcast address is the very last
address in this subnet’s range. This is a
Class B address which means we must
consider the last 2 octets
Host Address Range:

177.56.32.1 – 177.56.47.254
NOTE: This is a Class B address which
means we must consider the last 2 octets
Boolean AND
• ANDing is used by devices to determine which
Subnet an address belongs to
•
Devices AND IP addresses and their subnet
mask, to determine the subnet address
ANDing
= 10101100 00010000
= 172
16
00000010 00000000
2
this host is on the subnetwork 172.16.2.0
0
Reverse Subnetting
You can also use Reverse Subnetting to determine
what subnet a host address belongs to
IP Host Address:
192.168.10.70
Default Subnet Mask:
255.255.255.224
Class :
C
Default Subnet Mask
255.255.255.0
How many Bits have been borrowed
(subnetted):
3
By what increment do the subnets
increase?
By 32
What is the subnet address?
192.168.10.64