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IPv4 SubNetting Cheat Sheet

Lately, I have been reading CCNA 640-802 Official Cert Library by Wendell Odom (a lot) as I am preparing to take my exam in a few months. It is a lot of information to know and the exam covers a lot of information, it is like CompTIA’s Network+ test, but it adds more content to master. You must know how to subnet very well and get to the point that you can already see the numbers when you see the mask. Then you must master the Cisco IOS commands, plus know how to trouble basic network problems with hosts, switches and routers. 

A few months ago, I was very surprised when my manager would have me take this class, since my background is more system and/or database administration. To be honest, I was actually annoyed with this requirement to go get the certification, since I was already preparing to upgrade my Microsoft certifications to 2012 versions of Windows and SQL. Now, that I am preparing for the test and learning more about the networking side of things, I get why my manager wants me to get it done. More importantly, I see the value it adds to my career and how helpful it has become in my day to day job abilities.

For most of my career, I have been somewhat shielded from the network operation side of things. I knew what the OSI model was, but I did not fully grasp the encapsulation process of packets and how they go across the wire. Now, that I have been reading more, I see that it is a very helpful thing to know, because it is good to know what happens to your data when it leaves your server to fulfill a client request. Let’s be honest, it does no good to have awesome data if users can never access it. I will say that at this point, this has been the hardest exam to prepare for in my career, but I know I will master it.

So with that introduction, I would like to present a set of nifty cheat sheets that I created for IPv4 SubNetting:

Terminology

CIDR: Classless interdomain routing was developed to provide more granularity than legacy classful addressing. CIDR notation is expressed as /XX (bits)

For example, a full address is made up of 4 octates of 8 bytes per for a grand total of 32 bits as an address. A common subnet in a home network is the 255.255.255.0 or /24 mask. The first three 255 makes a set of 24 bits
turned on and the last 8 bytes are reserved for the network hosts, which also include a network ID and broadcast. (Both of which are not usable)

VLSM: Variable-length subnet masks are an arbitrary length between 0 and 32 bits. CIDR relies on VLSM’s to define routes.

Subnets

CIDR Subnet Mask Addresses Wildcard Power of 2
/32 255.255.255.255

1

0.0.0.0

0

/31 255.255.255.254

2

0.0.0.1

1

/30 255.255.255.252

4

0.0.0.3

2

/29 255.255.255.248

8

0.0.0.7

3

/28 255.255.255.240

16

0.0.0.15

4

/27 255.255.255.224

32

0.0.0.31

5

/26 255.255.255.192

64

0.0.0.63

6

/25 255.255.255.128

128

0.0.0.127

7

/24 255.255.255.0

256

0.0.0.255

8

/23 255.255.254.0

512

0.0.1.255

9

/22 255.255.252.0

1,024

0.0.3.255

10

/21 255.255.248.0

2,048

0.0.7.255

11

/20 255.255.240.0

4,096

0.0.15.255

12

/19 255.255.224.0

8,192

0.0.31.255

13

/18 255.255.192.0

16,384

0.0.63.255

14

/17 255.255.128.0

32,768

0.0.127.255

15

/16 255.255.0.0

65,536

0.0.255.255

16

/15 255.254.0.0

131,072

0.1.255.255

17

/14 255.252.0.0

262,144

0.3.255.255

18

/13 255.248.0.0

524,288

0.7.255.255

19

/12 255.240.0.0

1,048,576

0.15.255.255

20

/11 255.224.0.0

2,097,152

0.31.255.255

21

/10 255.192.0.0

4,194,304

0.63.255.255

22

/9 255.128.0.0

8,388,608

0.127.255.255

23

/8 255.0.0.0

16,777,216

0.255.255.255

24

/7 254.0.0.0

33,554,432

1.255.255.255

25

/6 252.0.0.0

67,108,864

3.255.255.255

26

/5 248.0.0.0

134,217,728

7.255.255.255

27

/4 240.0.0.0

268,435,456

15.255.255.255

28

/3 224.0.0.0

536,870,912

31.255.255.255

29

/2 192.0.0.0

1,073,741,824

63.255.255.255

30

/1 128.0.0.0

2,147,483,648

127.255.255.255

31

/0 0.0.0.0

4,294,967,296

255.255.255.255

32

Please note that even though these numbers look like they are decimal (a.k.a powers of 10) they are really binary numbers. (a.k.a. powers of 2) You will need to realize that important point when you start to subnet, because you will see that you do not know how to count until you master that concept. J

Decimal to Binary

Subnet Mask

Binary

Wildcard

Binary

255

1111 1111

0

0000 0000

254

1111 1110

1

1000 0000

252

1111 1100

3

1100 0000

248

1111 1000

7

1110 0000

240

1111 0000

15

1111 0000

224

1110 0000

31

1111 1000

192

1100 0000

63

1111 1100

128

1000 0000

127

1111 1110

0

0000 0000

255

1111 1111

Helpful hint: I use my eight fingers to count in binary and I start with my left hand with all four fingers up and my right hand has all four fingers closed (as in making a fist) to symbolize a 240 mask. I also use that to help me see the sets of # when determining networks. The 240 is sets of 16. One finger up is 128 mask with sets of 128. Two fingers up is 192 mask with sets of 64. Three fingers up is 224 mask with sets of 32. Four fingers up is 240 mask with sets of 16.

Classful Ranges

Class Range Purpose # of Network Bytes # of Hosts Bytes # of Addresses per network
A 1.0.0.0 – 126.255.255.255 Unicast (Large Networks)

1 (8 Bits)

3 (24 Bits)

2^24 -2 (16,777,214)
B 128.0.0.0 – 191.255.255.255 Unicast (Medium Networks)

2 (16 Bits)

2 (16 Bits)

2^16 -2 (65,534)
C 192.0.0.0 – 223.255.255.255 Unicast Small Networks)

3 (24 Bits)

1 (8 Bits)

2^8 -2 (254)
D 224.0.0.0 – 239.255.255.255 Multicast

 

   
E 240.0.0.0 – 255.255.255.255 Experimental

 

   

 

Reserved Ranges

RFC 1918 10.0.0.0 – 10.255.255.255
Localhost (Loopback) 127.0.0.0 – 127.255.255.255
RFC 1918 172.16.0.0. – 172.31.255.255
RFC 1918 192.168.0.0 – 192.168.255.255

To help me calculate subnetting ranges manually, I have used this web site to help me get the network id, first valid IP, last valid IP and broadcast. When that becomes easy for you then you could go to a subnettingquestions.com to see the same concept, but asked in different method.

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3 Responses to “IPv4 SubNetting Cheat Sheet”

  1. Scott,

    I have been waiting for something like this for a long time. Thank you for publishing this page.

    xoxoxo,

    Belinda

    June 13, 2013 at 3:19 PM
  2. Wow! I can not say enough about how much this helps. I am in CNST 123,and I will definitely share this with my fellow students! Thanks A ton!

    October 13, 2013 at 10:16 PM

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