CCNA 200-301 Day 13

Subnetting (Part 1)
 Things we’ll cover

●
CIDR (Classless Inter-Domain Routing)
●
The process of subnetting
 IPv4 Address Classes

First octet First octet range

Class
(binary) (decimal)
0.0.0.0 ~
A 0xxxxxxx 0 - 127 127.255.255.255

128.0.0.0 ~
B 10xxxxxx 128 - 191 191.255.255.255

192.0.0.0 ~
C 110xxxxx 192 - 223 223.255.255.255

224.0.0.0 ~
D 1110xxxx 224 - 239 239.255.255.255

240.0.0.0 ~
E 1111xxxx 240 - 255 255.255.255.255
 IPv4 Address Classes

First octet Prefix

Class First octet
numeric range Length

A 0xxxxxxx 0-127 /8

B 10xxxxxx 128-191 /16

C 110xxxxx 192-223 /24

IPv4 Address Classes
 IPv4 Address Classes

●
The IANA (Internet Assigned Numbers Authority) assigns
IPv4 addresses/networks to companies based on their size.
●
For example, a very large company might receive a class A
or class B network, while a small company might receive a
class C network.
●
However, this led to many wasted IP addresses.
 IPv4 Address Classes

San Francisco New York

point-to-point network

R1 R2
 IPv4 Address Classes

San Francisco New York

203.0.113.0/24

.1 .2

R1 R2
256 addresses
-1 network address (203.0.113.0)
-1 broadcast address (203.0.113.255) =252 addresses
-1 R1’s address (203.0.113.1) WASTED
-1 R2’s address (203.0.113.2)
 IPv4 Address Classes

●
Company X needs IP addressing for 5000 end hosts.
●
A class C network does not provide enough addresses, so a
class B network must be assigned.
●
This will result in about 60000 addresses being wasted.
 CIDR (Classless Inter-Domain Routing)
●
When the Internet was first created, the creators did not
predict that the Internet would become as large as it is
today.
●
This resulted in wasted address space like the examples I
showed you (there are many more examples).
●
The IETF (Internet Engineering Task Force) introduced CIDR
in 1993 to replace the ‘classful’ addressing system.
 CIDR (Classless Inter-Domain Routing)
●
With CIDR, the requirements of...
Class A = /8
Class B = /16
Class C = /24
...were removed.
●
This allowed larger networks to be split into smaller
networks, allowing greater efficiency.
●
These smaller networks are called ‘subnetworks’ or
‘subnets’.
 CIDR
203.0.113.0/24

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 0 0 0 0 0 0 0 0

255 . 255 . 255 . 0

network address, broadcast address

28 – 2 = 254 usable addresses.

number of host bits
 CIDR Practice!

How many usable addresses are there in each network?

●
203.0.113.0/25
●
203.0.113.0/26
●
203.0.113.0/27 2n – 2 = usable addresses
●
203.0.113.0/28 n = number of host bits
●
203.0.113.0/29
●
203.0.113.0/30
●
203.0.113.0/31
●
203.0.113.0/32
 CIDR (/25)
203.0.113.0/25

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 0 0 0 0 0 0 0

255 . 255 . 255 . 128

27 – 2 = 126 usable addresses.

 CIDR (/26)
203.0.113.0/26

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 0 0 0 0 0 0

255 . 255 . 255 . 192

26 – 2 = 62 usable addresses.
 CIDR (/27)
203.0.113.0/27

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 0 0 0 0 0

255 . 255 . 255 . 224

25 – 2 = 30 usable addresses.
 CIDR (/28)
203.0.113.0/28

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 0 0 0 0

255 . 255 . 255 . 240

24 – 2 = 14 usable addresses.
 CIDR (/29)
203.0.113.0/29

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 0 0 0

255 . 255 . 255 . 248

23 – 2 = 6 usable addresses.
 CIDR (/30)
203.0.113.0/30

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 0 0

255 . 255 . 255 . 252

22 – 2 = 2 usable addresses.
 CIDR (/30)
203.0.113.0/30
.1 .2

203.0.113.0/30
= 203.0.113.0 – 203.0.113.3
1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0
1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 1
1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 1 0
1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 1 1

The remaining addresses in the 203.0.113.0/24 address block

(203.0.113.4 – 203.0.113.255) are now available to be used
in other subnets!
 CIDR (/31)
203.0.113.0/31

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 0

255 . 255 . 255 . 254

21 – 2 = 0 usable addresses.
 CIDR (/31)
203.0.113.0/31
.0 .1

203.0.113.0/30
= 203.0.113.0 – 203.0.113.1
1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0
1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 1

The remaining addresses in the 203.0.113.0/24 address block

(203.0.113.2 – 203.0.113.255) are now available to be used
in other networks!
 x
CIDR (/32)
203.0.113.0/32

1 1 0 0 1 0 1 1 . 0 0 0 0 0 0 0 0 . 0 1 1 1 0 0 0 1 . 0 0 0 0 0 0 0 0

203 . 0 . 113 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1

255 . 255 . 255 . 255

20 – 2 = -1 usable addresses?
 CIDR Notation

Dotted Decimal CIDR Notation

255.255.255.128 /25

255.255.255.192 /26

255.255.255.224 /27

255.255.255.240 /28

255.255.255.248 /29

255.255.255.252 /30

255.255.255.254 /31

255.255.255.255 /32
 Subnetting

203.0.113.0/31
.0 .1
 Subnetting

45 hosts
47 * 4 = 188 45 hosts

SW1 SW3

R1
45 hosts 45 hosts

SW2 SW4

192.168.1.0/24
Divide the 192.168.1.0/24 network into four subnets
that can accommodate the number of hosts required.
 Subnetting /30

1 1 0 0 0.0.0.0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
192 . 168 . 1 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 0 0

255 . 255 . 255 . 252

2 – 2 = 2 usable addresses
2

2*2=4
 Subnetting /29

1 1 0 0 0.0.0.0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
192 . 168 . 1 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 0 0 0

255 . 255 . 255 . 248

2 – 2 = 6 usable addresses
3

2*2*2=8
 Subnetting /28

1 1 0 0 0.0.0.0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
192 . 168 . 1 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 0 0 0 0

255 . 255 . 255 . 240

2 – 2 = 14 usable addresses
4

2 * 2 * 2 * 2 = 16
 Subnetting /27

1 1 0 0 0.0.0.0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
192 . 168 . 1 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 0 0 0 0 0

255 . 255 . 255 . 224

2 – 2 = 30 usable addresses
5

2 * 2 * 2 * 2 * 2 = 32
 Subnetting /26

1 1 0 0 0.0.0.0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 0
192 . 168 . 1 . 0
1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 0 0 0 0 0 0

255 . 255 . 255 . 192

2 – 2 = 62 usable addresses
6

2 * 2 * 2 * 2 * 2 * 2 = 64
 QUIZ
The first subnet (Subnet 1) is 192.168.1.0/26. What are the remaining
subnets?
HINT: Find the broadcast address of Subnet 1. The next address is the
network address of Subnet 2. Repeat the process for Subnets 3 and 4.

Subnet 1: Subnet 3:
192.168.1.0/26 ( )/26

Subnet 2: Subnet 4:
( )/26 ( )/26

192.168.1.0/24
 Things we covered

●
CIDR (Classless Inter-Domain Routing)
●
The process of subnetting (basics!)
 Supplementary Materials

●
Review flash cards
(link in the description)
JCNP-Level Channel Members
JCNP-Level Channel Members