Given a standard class B address, design a subnetting scheme to support 4 offices, a Central office needing 23 subnets and 200 hosts, Branch A with 8 subnets and 50 hosts, Branch B with 11 subnets and 50 hosts, and Branch C with 12 subnets and 150 hosts. No one office will have any more than 200 hosts needed for future growth.
Step 1:
First we need to add up the total number of subnets needed.
23
12
11
+ 8
57 subnets needed
Step 2:
Knowing we need to borrow bits from the left from our given standard class B address (172.16.0.0 /16) and that we need at least 57 subnets we can use our 2 to the n scale.
We see that to be able to assign at least 57 subnets we need to go to the next highest available of 64 and that we need to borrow 6 bits from our host side of the address using the subnet chart. At the 6th bit we then draw a line to see that our block size is 4 and our mask is 252 or /22.
Step 3:
We can now build our subnet addressing scheme starting with the office that needs the largest number of subnets and hosts which is the Central Office needing 23 subnets. Knowing we will be using block sizes of 4 from above we will start at our beginning address of 172.16.0.0 and increment each subnet in groups of 4. We can also see that we have 10 bits still available for hosts which by using the 2n scale shows us we have 1024 hosts or 1022 assignable hosts, far more than our needed 200 hosts per subnet but it does fit our parameters.
Our Central Office subnets = 172.16.0.0 – 172.16.88.0
Next largest is Branch C which needs 12 subnets and 150 hosts. Starting at the next available subnet of 172.16.92.0 we will continue.
Our Branch C subnets = 172.16.92.0 – 172.16.136.0
Next largest is Branch B which needs 11 subnets and 50 hosts. Starting at the next available subnet of 172.16.140.0 we will continue.
Our Branch B subnets = 172.16.140.0 – 172.16.180.0
Next largest is Branch A which needs 8 subnets and 50 hosts. Starting at the next available subnet of 172.16.184.0 we will continue.
Our Branch A subnets = 172.16.184.0 – 172.16.212.0
We then just need one subnet each per serial link.
Branch C will be 172.16.216.0
Branch B will be 172.16.220.0
Branch A will be 172.16.224.0
Chris Nadeau 
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