Network+ Guide to Networks, 8th Edition 8-1
Chapter 8
Subnets and VLANs
At a Glance
Instructor’s Manual Table of Contents
• Overview
• Objectives
• Teaching Tips
• Quick Quizzes
• Class Discussion Topics
• Additional Projects
• Additional Resources
• Key Terms
Network+ Guide to Networks, 8th Edition 8-2
Lecture Notes
Overview
Network segmentation takes the divide-and-conquer approach to network management. When
done well, it increases both performance and security on a network. A network can be
segmented physically by creating multiple LANs or logically through the use of VLANs
(virtual LANs). Either way, the larger broadcast domain is divided into smaller segments, and
the IP address space is subdivided as well.
In this chapter, the student will learn about two important concepts that enable and support
network segmentation: subnets and virtual LANs (or VLANs). Fundamentally, a subnet
Chapter Objectives
After reading this chapter and completing the exercises, the student will be able to:
• Explain the purposes of network segmentation
• Calculate and implement subnets
• Explain how VLANs work and how they’re used
Teaching Tips
Network Segmentation
1. Discuss the three reasons why a network administrator might separate traffic:
a. enhance security
2. Explain that networks are commonly segmented according to one of the following
groupings:
3. Use Figure 8-1 to help students visualize the concept of network segmentation.
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Subnets
2. Use Figure 8-3 to demonstrate how to segment the network from Figure 8-2 into three
3. Explain that using well-chosen subnets provide the following benefits:
a. Network documentation is easier to manage
b. Problems are easier to locate and resolve
How Subnet Masks Work
1. Explain how a device uses a subnet mask to determine which subnet or network it
2. Review the Application Concepts: Binary Calculation section with your students.
3. Review the Legacy Networking: Classful Addressing in IPv4 section with your
CIDR (Classless Interdomain Routing)
2. Define and describe a CIDR block.
IPv4 Subnet Calculations
1. Explain that subnetting alters the rules of classful IPv4 addressing and is sometimes
called classless addressing.
2. Discuss how to create a subnet by borrowing bits that would represent host information
3. Refer back to Figure 8-2 to discuss the number of bits used for network information:
4. Introduce the topic of calculating IPv4 subnets.
5. Work through the subnetting example starting on page 447 of the text in order to
demonstrate performing calculations using formulas.
a. Use Table 8-5 to show subnet information for eight possible subnets in a sample
IPv4 Class C network.
Point out that several online sites and operating systems provide calculators that
Subnet Mask Tables
1. Point out that each class reserves a different number of bits for network information so
2. Use Tables 8-6 and 8-7 to show students possible Class B and Class C subnet masks.
3. been divided into six (eight possible) subnetworks.
administrators in calculating subnet information. Provide a classroom
Point out that several Web sites provide excellent tools to assist network
Network+ Guide to Networks, 8th Edition 8-5
Subnetting Questions on Exams
1. Point out that students are likely to see two types of subnet calculation problems on the
CompTIA Network+ exam:
2. Have students work through the Applying Concepts: Calculate IPv4 Subnets and Host
Implement Subnets
1. Use Figure 8-7 to discuss how to implement three of the possible eight subnets listed
earlier in Table 8-5.
3. Review the steps on page 456, which outline the process of a DHCP server providing
VLSM (Variable Length Subnet Mask)
1. Explain that VLSM allows subnets to be further subdivided into smaller and smaller
groupings until each subnet is about the same size as the necessary IP address space.
2. Use the pizza analogy on page 457 to understand how VLSM works.
3. Use Table 8-8 and steps starting on page 458 of text to discuss configuring subnets
Subnets in IPv6
1. Explain that IPv6 addresses are classless and IPv6 does not use subnet masks.
3. Use Figure 8-12 to show the prefix and interface portions of an IPv6 address.
4. Remind students that they may see IPv6 addresses containing a slash, such as
Network+ Guide to Networks, 8th Edition 8-6
portion of the address.
5. Explain how subnet prefixes may be assigned from a (RIR) regional Internet registry
down to a local ISP level, using Figure 8-13.
Quick Quiz 1
1. True or False: A network administrator might separate traffic in order to enhance
security.
2. CIDR notation takes the network ID or a host’s IP address and follows it with a symbol
followed by the number of bits used for the network ID. Which symbol is used?
a. \
b. !
c. /
d. *
3. A centrally managed DHCP server can provide DHCP assignments to multiple subnets
with the help of which of the following?
a. subnet mask
b. variable length subnet mask
c. DHCP relay agent
d. CIDR notation
4. True or False: Subnet masks are only used in IPv4 classful addressing.
5. True or False: There are no IPv6 equivalents to IPv4’s Class A, Class B, or Class C
networks.
VLANs (Virtual Local Area Networks)
1. Define a VLAN (virtual local area networks). Point out that the end goal of
implementing a VLAN is so that routers can better manage network traffic.
2. Use Figure 10-25 to illustrate a simple VLAN design.
Network+ Guide to Networks, 8th Edition 8-7
3. Describe the advantages and reasons for using a VLAN:
a. Isolating connections with heavy or unpredictable traffic patterns
b. Identifying groups of devices whose data should be given priority handling
Managed Switches
1. Explain that managed switches can be configured via a command-line interface or a
3. Compare Figure 8-19 to Figure 8-18. Explain that in Figure 8-19, a managed switch is
used to separate two VLANs.
4. Introduce students to 802.1Q, which is the IEEE standard that specifies how VLAN
Switch Ports and Trunks
1. Explain that each port on a switch that supports VLANs is configured as one of two
2. Define the term trunking.
3. Define the term trunk.
Network+ Guide to Networks, 8th Edition 8-8
Teaching
Students may find more information on Understanding VLAN Trunk Protocol
(VTP) from Cisco at:
VLANs and Subnets
1. Explain that in most situations, each VLAN is assigned its own subnet of IP addresses.
Use Figure 8-25 to demonstrate.
2. Discuss the following rule:
Types of VLANs
1. Discuss the following types of VLANs:
a. default VLAN
b. native VLAN
View Configured VLANs
1. Use Figure 8-27 to illustrate the result of the show vlans command on a Cisco
Troubleshoot and Secure VLANs
1. Explain that configuration errors are a common cause of VLAN problems. Discuss
some of the common errors such as:
2. Discuss a VLAN hopping attack. Point out that there are two approaches to VLAN
hopping:
3. Discuss the following mitigation efforts that will reduce the risk of VLAN hopping:
a. Don’t use the default VLAN
b. Change the native VLAN to an unused VLAN ID
c. Disable auto-trunking on switches that don’t need to support traffic from
Quick Quiz 2
1. True or False: A VLAN groups ports on a Layer 2 switch so that some of the local
traffic on the switch is forced to go through a router.
2. To identify the transmissions that belong to each VLAN, the switch adds which of the
following to Ethernet frames?
a. tag
b. VLANID
c. trunk
d. port ID
3. True or False: An unmanaged switch can be configured via a command-line interface or
a web-based GUI.
4. Which of the following types of VLANs receives all untagged frames from untagged
ports?
a. default VLAN
b. native VLAN
c. data VLAN
d. management VLAN
5. Which IEEE standard specifies how VLAN information appears in frames and how
switches interpret that information?
a. 802.16
b. 802.1Q
c. 802.11ac
d. 802.3
Network+ Guide to Networks, 8th Edition 8-10
Class Discussion Topics
1. Discuss the benefits of subnetting.
2. Discuss why an organization would want to develop an enterprise-wide approach to
implementing VLANs.
Additional Projects
1. Provide students an example of a large network, currently with one router, one network
2. Have students research Cisco’s VTP (VLAN Trunk Protocol). Students should write at
least two reasons why VTP is the most popular protocol for exchanging VLAN
information over trunks.
Additional Resources
1. IP Subnet Calculator:
2. Understanding IP Addresses, Subnets, and CIDR Notation for Networking:
3. Segmenting a Network Using VLANs:
4. Understanding VLAN Trunk Protocol:
5. Basic VLAN Configuration:
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Key Terms
For definitions of key terms, see the Glossary near the end of the book.
➢ ip helper-address
➢ magic number
➢ managed switch
➢ native VLAN
➢ native VLAN mismatch