Networking – CIDR ( Classless Inter-Domain Routing )

Hello Friends,

This is continuation of my last blog about Subnet. In my last blog, I explain about what is subnet and classful notations of subnet.

Today, we will discuss about the CIDR ( Classless Inter-Domain Routing ). A system called Classless Inter-Domain Routing, or CIDR, was developed as an alternative to traditional subnetting. The idea is that you can add a specification in the IP address itself as to the number of significant bits that make up the routing or networking portion.

For example, we could express the idea that the IP address 192.168.0.15 is associated with the netmask 255.255.255.0 which is class- C by using the CIDR notation of 192.168.0.15/24. This means that the first 24 bits of the IP address given are considered significant for the network routing.

This allows us some interesting possibilities. We can use these to reference “supernets”.

Using CIDR notation, we can combine these blocks by referencing this chunk as 192.168.0.0/23. This specifies that there are 23 bits used for the network portion that we are referring to. We can divide it into more Subnets by using host bits.

So the first network (192.168.0.0/24) could be represented like this in binary:

1100 0000 – 1010 1000 – 0000 0000 – 0000 0000

While the second network (192.168.1.0/24) would be like this:

1100 0000 – 1010 1000 – 0000 0001 – 0000 0000

Where BROWN color bits are network bits which are fixed and GREEN color bits are host bits.

The CIDR address we specified indicates that the first 23 bits are used for the network block we are referencing. This is equivalent to a netmask of 255.255.254.0, or:

1111 1111 – 1111 1111 – 1111 1110 – 0000 0000

As you can see, with this block the 24th bit can be either 0 or 1 and it will still match, because the network block only cares about the first 23 digits. Basically, CIDR allows us more control over addressing continuous blocks of IP addresses. This is much more useful than the subnetting we talked about originally.

Please see below table which provide you details about CIDR notation with number of host you get with perticular CIDR notation.

IPv4 CIDR IP/CIDR

Δ to last IP addr

Mask

Hosts (*)

Class

IP/NW Bits

a.b.c.d/32

+0.0.0.0

255.255.255.255

1

1/256 C

a.b.c.d/31

+0.0.0.1

255.255.255.254

2

1/128 C

a.b.c.d/30

+0.0.0.3

255.255.255.252

4

1/64 C

a.b.c.d/29

+0.0.0.7

255.255.255.248

8

1/32 C

a.b.c.d/28

+0.0.0.15

255.255.255.240

16

1/16 C

a.b.c.d/27

+0.0.0.31

255.255.255.224

32

1/8 C

a.b.c.d/26

+0.0.0.63

255.255.255.192

64

1/4 C

a.b.c.d/25

+0.0.0.127

255.255.255.128

128

1/2 C

a.b.c.0/24

+0.0.0.255

255.255.255.000

256

1 C

a.b.c.0/23

+0.0.1.255

255.255.254.000

512

2 C

a.b.c.0/22

+0.0.3.255

255.255.252.000

1,024

4 C

a.b.c.0/21

+0.0.7.255

255.255.248.001

2,048

8 C

a.b.c.0/20

+0.0.15.255

255.255.240.000

4,096

16 C

a.b.c.0/19

+0.0.31.255

255.255.224.000

8,192

32 C

a.b.c.0/18

+0.0.63.255

255.255.192.000

16,384

64 C

a.b.c.0/17

+0.0.127.255

255.255.128.000

32,768

128 C

a.b.0.0/16

+0.0.255.255

255.255.000.000

65,536

256 C = 1 B

a.b.0.0/15

+0.1.255.255

255.254.000.000

131,072

2 B

a.b.0.0/14

+0.3.255.255

255.252.000.000

262,144

4 B

a.b.0.0/13

+0.7.255.255

255.248.000.000

524,288

8 B

a.b.0.0/12

+0.15.255.255

255.240.000.000

1,048,576

16 B

a.b.0.0/11

+0.31.255.255

255.224.000.000

2,097,152

32 B

a.b.0.0/10

+0.63.255.255

255.192.000.000

4,194,304

64 B

a.b.0.0/9

+0.127.255.255

255.128.000.000

8,388,608

128 B

a.0.0.0/8

+0.255.255.255

255.000.000.000

16,777,216

256 B = 1 A

a.0.0.0/7

+1.255.255.255

254.000.000.000

33,554,432

2 A

a.0.0.0/6

+3.255.255.255

252.000.000.000

67,108,864

4 A

a.0.0.0/5

+7.255.255.255

248.000.000.000

134,217,728

8 A

a.0.0.0/4

+15.255.255.255

240.000.000.000

268,435,456

16 A

a.0.0.0/3

+31.255.255.255

224.000.000.000

536,870,912

32 A

a.0.0.0/2

+63.255.255.255

192.000.000.000

1,073,741,824

64 A

a.0.0.0/1

+127.255.255.255

128.000.000.000

2,147,483,648

128 A

0.0.0.0/0

+255.255.255.255

000.000.000.000

4,294,967,296

256 A

Thanks for reading 🙂

Keep reading, share your thoughts, experiences. Feel free to contact us to discuss more. If you have any suggestion / feedback / doubt, you are most welcome.

Stay tuned on Knowledge-Junction, will come up with more such articles.

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