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IP Subnetting from Beginning to Mastery

IP Subnetting for IT Pros and Cisco CCNA Candidates

Wendell Odom

Do you want to stop worrying about IPv4 subnetting and instead make IPv4 subnetting one of your strengths? Then come to this course, and come ready to dig in, practice, and learn.

The need to master IPv4 subnetting may be the single most critical skill when designing, engineering, and operating TCP/IP networks. IPv4 subnetting influences many aspects of how IP works, including routing, routing protocols, and IP addressing. Additionally, subnetting requires the most effort, time, and practice to master compared to most TCP/IP topics due to the details of the processes and related math.

This course takes you through each area of subnetting to a depth useful for IT Pros and even deeper for certification candidates:

  • Learn subnetting: Understand the ideas and terms in each part of IPv4 subnetting, how to use the ideas in the related mathematical processes, and the purpose and meaning of the facts learned by those processes.
  • Do subnetting: First understand the why and how of each subnetting process, and then practice – with the opportunity to make mistakes and get your questions answered during the course.
  • Go faster at subnetting: For those preparing for Cisco CCNA or other certification exams, you need to be ready to do the math quickly. The course discusses how to practice for speed and how to set reasonable speed expectations.

The end goals of this course are simple: for you to know subnetting, be ready to talk subnetting, to be able to do subnetting math, and to be on the path to becoming fast enough at the math (without a calculator) to do well on the Cisco CCENT and CCNA exams.

What you'll learn-and how you can apply it

  • Identify and describe IPv4 addressing and subnetting processes that can be used to reveal facts about a TCP/IP subnet.
  • Perform a variety of subnetting math processes so that you get the correct answer each time, including: finding a host’s subnet ID, finding a subnet’s broadcast address, finding the range of IP addresses in the subnet, and manipulating subnet mask values into different formats.
  • Describe the details of the IPv4 address space, address classes, and predict facts about a class A, B, or C network.
  • Analyze and describe a corporate subnetting plan, using correct terms and math processes, about the number of subnets in the design, the number of host addresses in each subnet, and the possible subnet IDs per the design.
  • Describe how a subnet mask controls the specifics of a subnetting design.
  • Analyze the mask used in a design to determine the number of subnets created and the number of hosts per subnet.
  • Develop your plan for how to complete your study and improve your subnetting speed and accuracy for upcoming exams.

This training course is for you because...

  • You want to be better at your networking job, with a better understanding, better ability to troubleshoot, and a better ability to talk techie with your peers.
  • You want to pass any of the exams related to Cisco’s CCNA Routing and Switching Certification: The ICND1 100-105 Exam, the ICND2 200-105 Exam, and the CCNA 200-125 Exam.


  • Required: None
  • Useful: You will appreciate the subnetting topics of this class if you know the most basic concepts of IPv4 routing, routers, LANs, and WANs beforehand. You can get that level of knowledge from reading these chapters in the book, “ICND1 100-105 Official Cert Guide”. In particular:

  • Chapter 2: “Fundamentals of Ethernet LANs”

  • Chapter 3: “Fundamentals of WANs”

  • Chapter 4: “Fundamentals of IPv4 Addressing and Subnetting”

  • Chapter 11: “Implementing Ethernet Virtual LANs”, using only the section titled “Virtual LAN Concepts”

Course Setup

A subnetting calculator would be useful, but not required. You can then use the calculator to check your answers when practicing. Search the net for any subnetting calculator, install, and find out that you like. The calculator should let you type/select an IP address and mask, after which it should list the subnet ID, broadcast address, and range of usable IP addresses.


Optional: (video) Learning Path: CCNA Routing and Switching 200-125 Exam Prep

About your instructor

  • Wendell Odom, CCIE No. 1624, creates many of the best-selling Cisco certification products of their types, particular in the routing and switching space. He has pioneered the authorized Cisco Certification Guide series at Cisco Press and has written every edition of the leading CCENT and CCNA Certification Guides. He has written over 30 editions of networking books, video, and software products, ranging in depth from introductory level to CCIE. Wendell has worked as an instructor, course developer, network engineer, and consultant. Find links to more study tools and resources (including his blogs) at www.certskills.com.


The timeframes are only estimates and may vary according to how the class is progressing

Day 1: Operational Subnetting

Section 1: Analyzing Existing Subnets (2 hours)

What facts should you be able to determine about every IPv4 subnet that you see in a network? This section drills further into the different ideas about what a subnet is and what facts you might want to know about a subnet. Most of the time in this section is devoted to explaining and practicing the processes to discovers facts about any subnet.

  • Introducing Related Topics: IPv4 Addressing, Routing, and Routing Protocols
  • IPv4 Subnetting: Topologies, Concepts, and Math
  • Find the Subnet ID for the Easiest Cases
  • Find the Subnet ID (without a Calculator) for Difficult Cases
  • Practice

Section 2: Working with Existing Subnet Masks (45 minutes)

What facts should you be able to determine about every IPv4 subnet that you see in a network? This section drills further into the different ideas about what a subnet is and what facts you might want to know about a subnet. Most of the time in this section is devoted to explaining and practicing the processes to discovers facts about any subnet.

  • Understanding Mask Mechanics
  • Converting DDN Masks to Prefix Masks
  • Converting Prefix Masks to DDN Masks
  • Practice

Section 3: Understanding Public Networks, Private Networks, and CIDR Blocks (1 hour)

The first two sections each assume that someone had already created the subnets, with you then needing to understand those subnets. Section 3 steps back to an earlier step at each enterprise in which some network engineer chose to begin with some larger set of IPv4 addresses to then subnet the larger address block into the existing subnets. That larger address block could be a public IP network, a private IP network, and/or a CIDR block. This section examples the concepts, and then focuses on how to find key facts about a public or private network.

  • Examining the Entire IPv4 Address Space
  • Where Enterprises Begin: Public/Private Networks and CIDR Blocks
  • Listing Facts about a Classful Network
  • Calculating the Network ID (and Related Facts) of a Network
  • Practice

Day 2: Subnetting Design and Exam Prep Strategies

Office Hours: Review and QA about Day 1’s Content (20 minutes)

The first 20 minutes of the second day is set aside for your QA about the first day’s content – come ready to ask to see a practice problem worked again, or a concept explained again.

Section 4: Interpreting Subnet Designs and Plans (1:45 hours)

Day 2’s topics reveal more insights about the design choices, beginning with this section. This section explains how a network engineer views the options of different subnet masks, with a progressively longer mask creating twice as many subnets as the previous mask – but with each subnet then half as large. The section then goes on to show how you can determine all subnet IDs in a design, assuming all subnets use the same mask.

  • Noticing Patterns in a Subnetted Network
  • Understanding Classless Addressing and Classful Addressing
  • How to Create Design with Exactly 256 Subnets
  • How to Create Designs with Less Than 256 Subnets
  • Practice
  • How to Create Designs with More Than 256 Subnets
  • Practice

Section 5: Subnet Design Insights Through Subnet Masks (1:15 Hours)

The choice of subnet mask reveals key facts about the IPv4 subnetting plan: The number of subnets and the size of each subnet. This next section explains the related logic, and shows how to calculate the number of subnets and their size. This section also may include a topic about choosing a mask to meet design requirements, depending on the timing of this particular occurrence of the course.

  • Interpreting the Meaning of Existing Subnet Masks
  • Practice
  • The Impact of VLSM on the CCENT and CCNA Exams
  • Choosing Masks to Meet Design Requirements
  • Practice

Section 6: Exam Preparation Advice (30 minutes)

The final section examines two topics related to CCENT and CCNA Routing and Switching exam preparation, and then close with a few suggestions for continuing to improve your subnetting after the course.

  • CCENT and CCNA Exam Overview
  • Exam Day Plan for Subnetting
  • Practice Options after Class
  • Open Exam QA