Go through each example in turn. If necessary, draw the network and related facts on the whiteboard and then run through the process of answering each scenario.
Question: Contoso.com has implemented IPv4 throughout the organization. It is currently implementing a new head office building. The office will host 5,000 computers that are distributed fairly evenly across 10 floors of these offices. What address class would suit this scenario?
Answer: Any class would be suitable with CIDR; however, a class B network with subnetting is the most logical choice.
Question: Analysis of the network traffic at the existing head office shows that the maximum number of hosts for each subnet should be around 100. How many subnets are required, and assuming a network address for the whole site of 172.16.0.0/16, what mask should you use to ensure sufficient support for the required subnets?
Answer: At least 50 subnets are needed. To express 50 subnets, you will need 6 bits in the mask. 2^6 yields 64, while 2^5 provides only 32 subnets.
Question: Assuming the network address for the head office is 172.16.0.0/19, what mask would you assign to each subnet?
Answer: The mask would be 25 bits, and would be expressed in decimal as 255.255.255.128.
Question. How many hosts can you have in each subnet based on your selected mask?
Answer: There are 7 bits remaining for hosts, which allows for 2^7-2 hosts, which is 126. If the subnet mask was 26 bits, that would provide for only 62 hosts and the requirement is for a maximum of 100. However, a mask of 26 bits would support 128 subnets and can be considered as a valid configuration.
Question: Assuming you implement the mask that you determined for each subnet, what would the first subnet address be?
Answer: With a 6 bit subnet mask, the actual decimal mask would be 255.255.255.128. The first subnet would be 172.16.0.0/26. If you opted for a 7 bit mask, then again, the first subnet would be 172.16.0.0/27, but the decimal mask would be 255.255.255.192.
Question: What are the first and last host addresses for the first subnet?
Answer: With a 6 bit subnet mask, the first host in the first subnet would be 172.16.0.1/26 and the last host would be 172.16.0.126/26. Using a 7 bit mask, the first host would be 172.16.0.1/27, while the last host would be 172.16.0.62/27.