Here’s the crucial next step: To compute the range of available IP addresses, we start by subtracting whatever the value of the special octet is from 256. Every eight steps we take in CIDR notation, we will jump to the next octet. One step up in the CIDR notation (/23) moves the previous octet up to 254 (so, the subnet mask is 255.255.254.0). One step down in CIDR notation (/25, in this case) moves to the next octet and starts the count over again at 128 (so, the subnet mask would be 255.255.255.128). In our subnet masks, we always want to really pay attention to the “special octet” - the first grouping that does not equal 255. That “special” section (known as an octet) that equals zero in the subnet mask means there are 256 possible values for available IP addresses (it will only show up as 255 because 0 is actually the first value - thank you, computer programmers). The /24 notation is equal to a subnet mask of 255.255.255.0 - in many ways, this will be our “baseline” (so to speak) on which we will base all other calculations. However, if memorization is something you’re not great at, the main takeaway is this, right here: If this seems like too much to memorize, don’t worry - there is a system to everything here, and I will explain it at the bottom. The most important thing to remember is the following table: Of course, in an exam setting (and perhaps on the job), you’re timed, so I wanted to come up with a useful way to deal with CIDR notation and subnet masks without spending an eternity in calculations and double-checking all the 1’s and 0’s.Īs always, if you see an error in my post (or, especially, my math), please call me out - the whole point of this exercise is learning, after all. I’m going to assume you already understand the basic reason for subnetting, and, instead of going over the entire topic, I’ll share a simple way to arrive at the correct numbers. I spent the last week reviewing networking concepts in order to take the CompTIA Network+ exam ( I passed!), and, after reviewing subnets, I thought it might be helpful to share my process. If you’ve ever thought that, then this blog post is for you. “This should be easy! I know what an IP address is. To complicate matters, most online explanations for subnetting never really clicked for me - I (mostly) understood the theory of splitting up an address into multiple subnetworks, but who wants to spend an endless amount of time writing out the binary for every IP address and subnet mask? That’s not how anyone would spend their time in a real-world scenario where there are such things as time constraints and online calculators. I am notoriously bad at doing anything other than simple math in my head, and it always felt like I was one step behind understanding what I needed to do to get the right result. When I was first learning about networks, the topic that most consistently stumped me was subnetting.
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