Cisco Slash Notation

In this Cisco CCNA tutorial, you’ll learn how to write and convert the dotted decimal subnet mask into slash (/) notation. Scroll down for the video and also the text tutorial.


Cisco Slash Notation Video Tutorial

YouTube video

Olumide Olotu

Olumide Olotu

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Subnet Mask in Slash Notation


In the example below, the subnet mask is in decimal notation. We can write the subnet mask in binary, and you can see a line between the network and host portions. It's always contiguous 1's and then followed by a block of contiguous 0's. Because it's contiguous, we can count how many 1's that we have in a row.


Here, we have got 24 1's in a row. Each block is an octet (8), so that's 8, 16, 24. Therefore the dotted decimal notation can be written as /24 in slash notation. Those two things both mean the same thing.


Cisco Subnet Mask in Slash Notation


If we had as a subnet mask, we could write that as /16, and so on. Whenever you configure a Cisco router or switch on IOS, the IP address and the subnet mask configuration are written out in the full dotted decimal notation. But whenever we're having a conversation with somebody, or if we are creating a network diagram, more commonly, we will use slash notation.


It's not much fun to be saying all the time. It's much easier to say /24. Also, /24 takes up much less space on your network diagrams, so you can have your diagrams looking a lot neater and tidier. There's the network portion for a /24 highlighted.


Cisco Subnet Mask in Slash Notation 1


Subnet Mask in Slash Notation Example 2


Let's look at another example. We have IP address /, and hopefully, you can just by looking at this, you're going to see that that's going to be a /8. The more you get used to working with IP addresses and subnets, you'll very quickly be able to see where the lines are by just looking at the actual address and mask.


In the example below, the IP address can be written as or The network address is, not /8.


Subnet Mask in Slash Notation Example 2


When we write this out, it's just the network portion of the address that we specify. It's a /8, so it's only the first octet that includes the network portion, so that is the 10. All the rest of the address, the last three octets is the host portion.


When we write this address, it would be, and the available addresses would be for our first host. The last host would be We can't use because that would be the broadcast address, and is the network address.


Additional Resources


Subnet Mask:

Configure IP Addresses and Unique Subnets for New Users:

About Slash Notation:

Subnetting Practice Questions:


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Libby Teofilo

Text by Libby Teofilo, Technical Writer at

With a mission to spread network awareness through writing, Libby consistently immerses herself into the unrelenting process of knowledge acquisition and dissemination. If not engrossed in technology, you might see her with a book in one hand and a coffee in the other.