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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Batavia High School
Cisco Networking Academy
..make it a short-cut in the way you get something done, not a class!
Class B IP address example:
Network Bits Host Bits
The subnet mask would be:
Network Bits Host Bits
2N = total subnets needed
N = #bits borrowed = 8
28 = 256 total subnets
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
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
0 184.108.40.206 220.127.116.11 - 18.104.22.168 22.214.171.124
1 126.96.36.199 188.8.131.52 – 184.108.40.206 220.127.116.11
2 18.104.22.168 22.214.171.124 – 126.96.36.199 188.8.131.52
3 184.108.40.206 220.127.116.11 – 18.104.22.168 22.214.171.124
See any pattern here? What would SN 212 look like?
212 126.96.36.199 188.8.131.52 – 184.108.40.206 220.127.116.11
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
Let’s look at another example using a
Class C address:
IP Address = 18.104.22.168
SN Mask = 255.255.255.0
Need to create 14 subnets, will have to borrow 4 host bits (24 = 16).
New SN Mask = 255.255.255.240
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!