CIDR and calculating Subnets - griffinsnest/tech-Jorunal-1 GitHub Wiki
How to use CIDR to find the correct subnet
For this lab, we are only given the number of hosts needed for each subnet. So in this case we need to use CIDR and the number of hosts/usable IPs in these networks to figure out which one best fits the number needed. As an example for the Sales network, we needed 1200 usable hosts, so the closest subnet we have to that amount is /21. This subnet gives 2048 hosts to use, which may seem like a waste since that's over 800 unused addresses, the next closest one would /22 but that one only goes up to 1024 hosts so that doesn't have enough. For a list of the generic amount of hosts for given networks look at the first attached image.
For the calculating of the addresses it is pretty simple, for each of the CIDR types there is a set space/octet in the address that is incremented by a specific amount. In the case of the Sales example given previously, it has to begin with 172.16.0.0 since it will be the first to be subnetted as it is the largest. As a /21 subnet the 3rd octet, 172.16.0.0 is to be incremented, and in the case of a /21 subnet the increment is 8, so the next subnet has to begin at 172.16.8.0 and this subnet ends at 172.16.7.255. For a detailed list of the CIDR and the octet increment places and amounts see the second attached photo.
Repeat these steps to find the correct addresses for each of the subnets for the rest of the network. The full calculation sheet can be seen here:
