This week has focused on the Network layer. For one assignment, we had to determine the subnets and IP address allocations on a fictional network. Without any solid examples of how to work a problem like this out, I had to do a lot of extensive independent research to figure out how to ultimately solve this problem.

As usual, diagramming things out really helped me. You can find my diagram below:

Basically, given a network broken up into separate subnets, if you know how many interfaces each subnet should support, you can use CIDR to determine how many bits of a 32 bit IP address should be allocated to network addresses. In the example above, we are given that subnet A should support 250 interfaces. First, we determine how many bits are needed to represent 250 unique values. If we allocate 8 bits, that is 2^8 or 256 unique values (0 – 255). Subtracting those 8 bits from the total 32 bits allocated to an IP address, we get 24. This is the number of bits used for the subnet mask, written as /24.

Since we were given a starting address of 216.32.254.0, we know that the address of subnet A is 216.32.254.0/24 and that it supports IP addresses on the range 216.32.254.1 – 216.32.254.255.

Last, I was able to check my understanding with this CIDR subnet calculator!