Subnetting shortcuts
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Subnetting Shortcuts. Batavia High School Cisco Networking Academy Semester 3 Mr. Mullen. If you’re going to cut…. ..make it a short-cut in the way you get something done, not a class! Subnetting shortcuts are a way to: Determine subnet addresses

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Subnetting Shortcuts

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Subnetting shortcuts

Subnetting Shortcuts

Batavia High School

Cisco Networking Academy

Semester 3

Mr. Mullen


If you re going to cut

If you’re going to cut…

..make it a short-cut in the way you get something done, not a class!

  • Subnetting shortcuts are a way to:

    • Determine subnet addresses

    • Determine a specific host address within a specific subnet.

    • Take the computational confusion out of determining a subnet address range


Let s cut to the chase

Let’s cut to the chase!

Class B IP address example:

145.31.0.0

10010001.00011111.00000000.00000000

Network Bits Host Bits

The subnet mask would be:

255.255.0.0

11111111.11111111.00000000.00000000


Remember just a few things

Remember just a few things..

  • We’ll start small and easy, suppose we want to create a total of 256 SN’s.

  • Starting from the left, we need to borrow the 8 most significant host bits or the entire 3rd octet of the Network ID

    10010001.00011111.00000000.00000000

    Network Bits Host Bits

Subnet Bits


What is the result

What is the result?

  • The new subnet mask must reveal the borrowed bits:

    11111111.11111111.11111111.00000000

    255.255.255.0

    Borrowing rule:

    2N = total subnets needed

    N = #bits borrowed = 8

    28 = 256 total subnets


What about the of hosts

What about the # of hosts?

11111111.11111111.1111111.00000000

Remember there are 16 total host bits in a Class B, we have borrowed 8 for subnetting. That leaves us with 8 host bits.

Using the # of hosts rule:

2N = total hosts possible

N = #bits remaining = 8

28 = 256 total hosts possible/subnet

Subnet Bits

Host Bits


Cursive review so far right

Cursive review, so far, Right?

Let’s look at the first few subnets. Remember, the first subnet is not useable, but lets list it anyways.

SN #SNA Useable Range Broadcast

0 145.31.0.0 145.31.0.1 - 145.31.0.254 145.31.0.255

1 145.31.1.0 145.31.1.1 – 145.31.1.254 145.31.1.255

2 145.31.2.0 145.31.2.1 – 145.31.2.254 145.31.2.255

3 145.31.3.0 145.31.3.1 – 145.31.3.254 145.31.3.255

See any pattern here? What would SN 212 look like?

212 145.31.212.0 145.31.212.1 – 145.31.212.254 145.31.212.255


So what s the big deal nothing really

So what’s the big deal? Nothing really!

Let’s look at another example for the same address, but lets create 512 total subnets this time.

Subnets = 29 = 512

Hosts = 2(16-9) =27 = 128

SN host range #s:

.0 - .127 and .128 - 255


Subnetting shortcuts

And today’s winning SN #’s are…512 Total Subnets with 128 Possible Hosts\SN

Note: when the SN# is even, it is just 2x’s the value of the 3rd octet

Note: when the SN# is even, the value of the 4th octet begins at 0 and when SN# is odd, the value of the 4th octet begins at 128


How about this one

How about this one…

Let’s look at another example using a

Class C address:

IP Address = 207.142.25.0

SN Mask = 255.255.255.0

Need to create 14 subnets, will have to borrow 4 host bits (24 = 16).

11111111.11111111.11111111.11110000

New SN Mask = 255.255.255.240


Let s have some fun with the s

Let’s have some fun with the #’s

16 remember that #

# of Subnets = 2n = 24 = 16

# of Hosts = 24 = 16

Note: that the 4th octet value for the SNA is 16x’s the value SN#

Note: vertically all the blue numbers increase by a value of 16


Subnetting shortcuts

So the next time you are getting frustrated with subnetting….

Play around and

have some fun

with the numbers!


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