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

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

    • 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!

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..

  • 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?

  • 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?

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?

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!

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


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…

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

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


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

Play around and

have some fun

with the numbers!


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