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Course: IP Version 4 Addressing And Subnetting Deep Dive (Part 6)


3 months ago5 min read

Hello and welcome, I am currently working on a video course 'addressing' IP version 4, and I've decided to post my slides and scripts here on STEEM as I go along. I appreciate any feedback or suggestions, or if you are new to the world of computer networking, I hope you enjoy them!

Here is part 6 of a multi-part series. How many parts will there be? I have no idea because I am posting them as I go along. This section can be a bit repetitive, but you could skip through it fairly fast once you pick up the pattern of what happens to the number of networks and hosts as you add a bit to the subnet mask.

If you missed any sections, click one of the links below for your favorite front end:


From here I’m going to go through each of the /24 and longer subnet lengths and show you how they break down into subnets. It can be repetitive, but I think it’s essential to picking up the pattern and easily picturing subnets in your head.

First we have the ubiquitous /24 or Class C subnet. This is most common subnet to deploy from my experience. When you have plenty of addresses to work with, such as when you are using private addresses, you can worry less about being conservative with IP addresses. When you have a decent sized enterprise, and all you have to work with is a /24, subnetting is essential.

As we’ve covered previously, we the network address, subnet mask, broadcast and host address, and usable hosts for a /24. I’ve also broken the subnet mask down to binary, so you can see how it changes as we ad bits for subnetting. There are 254 useable addresses in a /24 because 2^8-2 is 254. At this point, we have an entire /24 pie, so let’s start slicing it up!


For a /25 we claim the first bit of the host address to be used for subnetting. This bit is indicated in green on the binary subnet mask. We know that the first bit represents 128 in decimal, so it creates a network address of either 0 or 128. This splits the /24 into two parts. Each part has 126 useable addresses because 2^7-2 = 128.


For a /26, we add another subnet bit (indicated in green). This splits the /24 into four parts. Each has 62 addresses, because 2^6-2 = 62. Do you see a pattern yet?


For a /27, we add yet another subnet bit (indicated in green). This splits the /24 into eight parts. Each has 30 addresses, because 2^5-2 = 30. Now the slide is starting to get crowded.


For a /28, we add yet another subnet bit (indicated in green). This splits the /24 into sixteen parts. Each has 14 addresses, because 2^4-2 = 14. I can’t fit them all here, you’re going to have to take my word for it.


For a /29, we add still another subnet bit (indicated in green). This splits the /24 into thirty-two parts. Each has 6 addresses, because 2^3-2 = 6. I can’t fit them all here, you’re really going to have to take my word for it.


For a /30, you know the deal. This splits the /24 into sixty-four parts. Each has 2 addresses, because 2^2-2 = 2. That’s a lot of slicing, but you are also wasting half of your IP addresses when they are transformed to network and broadcast addresses. A /30 is typically used for point-to-point links, or the extremely rare occasion where you need to have a network with a single IP address. Why do I say a single IP address when there are clearly 2 available? This is because that single device will need a default gateway to reach any other network. I normally recommend not to place a device such as a server onto a /30 network, because it leaves you no room for expansion. You will learn about default gateways a bit later.


Then we arrive at the /31. The /31 was standardized in 2000 in RFC 3021. It creates a subnet type the previously did not exist. The /31 slices a Class C network into 128 parts. The kicker is, there is no network or broadcast address. It is just two addresses. This subnet type is meant specifically for point-to-point Wide Area Network (WAN) links used for routing protocols to exchange information.


Finally we have a /32. A /32 really isn’t a subnet. It’s how we reference a single IP address, so a /24 or Class C can contain 256 of them. You can also use the .0 or .255 as host address when using a /32. A /32 is often used for adding an IP address to a loopback interface. A loopback interface is simply a logical interface on a device used exclusively for management and monitoring. In routing a /32 address and subnet mask is referred to as a host route, meaning you can create a network route pointing to a single IP address. IP routing is beyond the scope of this course.

Thanks for viewing. In the next part, we will go over subnetting networks larger than a /24, and how you can mix and match different length subnets within the same network block.


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