Prefix to Mask

Lesson 12 of 46 in Coddy's IP Addressing and Subnetting course.

Prefix to mask is the same walk in the other direction: lay down that many ones, fill the rest with zeros, and read off the four octets.

Take /20. Two full octets use 16 ones; 4 ones remain for the third octet: 11110000, which is 240. The mask is 255.255.240.0.

A sanity check that catches most mistakes: a mask can only ever contain the values 255, 254, 252, 248, 240, 224, 192, 128 or 0, and once an octet drops below 255, everything after it must be 0.

Challenge

Beginner

Convert each prefix to its dotted-decimal mask. One per line, in this order:

/20
/30

Expected output:

255.255.240.0
255.255.255.252

Try it yourself

Terminal

All lessons in IP Addressing and Subnetting

1Bits and Binary

Numbers as Bits Binary to Decimal Powers of Two Recap - Binary Warm-up

4CIDR Notation

Goodbye Classes Reading CIDR Block Sizes Recap - CIDR Reading

7Special Addresses

Private Ranges Loopback Link-Local Zero and Broadcast

10Capstone - The Office

The Floor Plan Sizing Each Team Carving the Block The Router's View Recap - Ship the Plan

2IPv4 Anatomy

Four Octets The 32-Bit View Network and Host Valid or Not Recap - Read an Address

5Inside a Subnet

Network Address Broadcast Address Usable Hosts First and Last Host Recap - Subnet Facts

8Planning Networks

Right-Sizing VLSM Step by Step Allocation Order Checking for Overlap Recap - Plan a Network

3The Subnet Mask

What a Mask Does Mask to PrefixPrefix to MaskANDing It Out Recap - Mask Basics

6Subnetting Drills

The Block Size Trick Which Subnet Am I In Splitting a /24 Splitting Again Recap - Subnetting Drills

9IPv6 First Steps

Why IPv6 Reading IPv6 Prefixes in IPv6 Recap - IPv6 Basics

Practice on your own: Terminal playground