Home > Articles > Cisco Certification > CCNA Routing and Switching > CCNA ICND2 Cert Guide: Variable Length Subnet Masks

CCNA ICND2 Cert Guide: Variable Length Subnet Masks

Sample Chapter is provided courtesy of Cisco Press.
Date: Dec 19, 2011.

Save Save Print

Chapter Information

Chapter Description

This chapter introduces VLSM, including VLSM Concepts and Configuration, Finding VLSM Overlaps, Adding New Subnets to an Existing VLSM Design, and Designing a Subnetting Plan Using VLSM.

From the Book

CCNA ICND2 640-816 Official Cert Guide, 3rd Edition

$39.99

Exam Preparation Tasks

Review All the Key Topics

Review the most important topics from this chapter, noted with the Key Topics icon in the outer margin of the page. Table 5-8 lists a reference of these key topics and the page numbers on which each is found.

Table 5-8. Key Topics for Chapter 5

Key Topic Element	Description	Page Number
Table 5-2	Classless and classful routing protocols listed and compared	203
List	Steps to analyze an existing design to discover any VLSM overlaps	206
List	Steps to follow when adding a new subnet to an existing VLSM design	209
Paragraph	Statement of the main VLSM subnet assignment strategy or assigning the largest subnets first	214
List	Steps to follow to design a subnet plan using VLSM	218

Complete the Tables and Lists from Memory

Print a copy of Appendix J, “Memory Tables,” (found on the DVD) or at least the section for this chapter, and complete the tables and lists from memory. Appendix K, “Memory Tables Answer Key,” also on the DVD, includes completed tables and lists to check your work.

Definitions of Key Terms

Define the following key terms from this chapter and check your answers in the Glossary:

classful routing protocol, classless routing protocol, overlapping subnets, variable length subnet masks (VLSM)

Read Appendix G Scenarios

Appendix G, “Additional Scenarios,” contains five detailed scenarios that both give you a chance to analyze different designs, problems, and command output and show you how concepts from several different chapters interrelate. Appendix G Scenario 1, Part A, and all of Scenario 5 provide an opportunity to practice and develop skills with VLSM.

Appendix D Practice Problems

Appendix D, “Practice for Chapter 5: Variable Length Subnet Masks,” lists additional practice problems and answers. You can find this appendix on the DVD as a printable PDF.

Answers to Earlier Practice Problems

Answers to Practice Finding VLSM Overlaps

This section lists the answers to the five practice problems in the section, “Practice Finding VLSM Overlaps,” as listed earlier in Table 5-4. Note that the tables that list details of the answer reordered the subnets as part of the process.

In Problem 1, the second and third subnet IDs listed in Table 5-9 happen to overlap. The second subnet’s range completely includes the range of addresses in the third subnet.

Table 5-9. VLSM Overlap Problem 1 Answers (Overlaps Highlighted)

Reference	Original Address and Mask	Subnet ID	Broadcast Address
1	10.1.1.1/20	10.1.0.0	10.1.15.255
2	10.1.17.1/21	`10.1.16.0`	`10.1.23.255`
3	10.1.23.254/22	`10.1.20.0`	`10.1.23.255`
4	10.1.29.101/23	10.1.28.0	10.1.29.255
5	10.1.34.9/22	10.1.32.0	10.1.35.255

In Problem 2, again, the second and third subnet IDs (listed in Table 5-10) happen to overlap, and again, the second subnet’s range completely includes the range of addresses in the third subnet. Also, the second and third subnet IDs are the same value, so the overlap is more obvious.

Table 5-10. VLSM Overlap Problem 2 Answers (Overlaps Highlighted)

Reference	Original Address and Mask	Subnet ID	Broadcast Address
1	172.16.122.1/30	172.16.122.0	172.16.122.3
2	172.16.122.57/27	`172.16.122.32`	`172.16.122.63`
3	172.16.122.33/30	`172.16.122.32`	`172.16.122.35`
4	172.16.126.151/22	172.16.124.0	172.16.127.255
5	172.16.128.151/20	172.16.128.0	172.16.143.255

In Problem 3, three subnets overlap. Subnet 1’s range completely includes the range of addresses in the second and third subnets. Note that the second and third subnets do not overlap with each other, so for the process in this book to find all the overlaps, after you find that the first two subnets overlap, you should compare the next entry in the table (3) with both of the two known-to-overlap entries (1 and 2).

Table 5-11. VLSM Overlap Problem 3 Answers (Overlaps Highlighted)

Reference	Original Address and Mask	Subnet ID	Broadcast Address
1	192.168.1.113/28	`192.168.1.112`	`192.168.1.127`
2	192.168.1.122/30	`192.168.1.120`	`192.168.1.123`
3	192.168.1.125/30	`192.168.1.124`	`192.168.1.127`
4	192.168.1.245/29	192.168.1.240	192.168.1.247
5	192.168.1.253/30	192.168.1.252	192.168.1.255

Answers to Practice Adding VLSM Subnets

This section lists the answers to the five practice problems in the section, “Practice Adding VLSM Subnets.”

All five problems for this section used the same set of five pre-existing subnets. Table 5-12 lists those subnet IDs and subnet broadcast addresses, which define the lower and higher ends of the range of numbers in each subnet.

Table 5-12. Pre-Existing Subnets for the Add a VLSM Subnet Problems in This Chapter

Subnet	Subnet Number	Broadcast Address
1	10.0.0.0/24	10.0.0.255
2	10.0.1.0/25	10.0.1.127
3	10.0.2.0/26	10.0.2.63
4	10.0.3.0/27	10.0.3.31
5	10.0.6.0/28	10.0.6.15

The rest of the explanations follow the five-step process outlined earlier in the section, “Finding VLSM Subnets,” except that the explanations ignore Step 3 because Step 3’s results in each case are already listed in Table 5-12.

Problem 1

Step 1 The problem statement tells us to use /24.

Step 2 The subnets would be 10.0.0.0, 10.0.1.0, 10.0.2.0, 10.0.3.0, 10.0.4.0, 10.0.5.0, and so on, counting by 1 in the third octet.

Step 3 The first four new possible subnets (10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24, and 10.0.3.0/24) all overlap with the existing subnets (see Table 5-12). 10.0.6.0/24 also overlaps.

Step 4 10.0.4.0/24 is the numerically lowest new subnet number that does not overlap with the existing subnets.

Problem 2

Step 1 The problem statement tells us to use /23.

Step 2 The subnets would be 10.0.0.0, 10.0.2.0, 10.0.4.0, 10.0.6.0, 10.0.8.0, and so on, counting by 2 in the third octet.

Step 3 Three of the first four new possible subnets (10.0.0.0/23, 10.0.2.0/23, and 10.0.6.0/23) all overlap with existing subnets.

Step 4 10.0.4.0/23 is the numerically lowest new subnet number that does not overlap with the existing subnets.

Problem 3

Step 1 The problem statement tells us to use /22.

Step 2 The subnets would be 10.0.0.0, 10.0.4.0, 10.0.8.0, 10.0.12.0, and so on, counting by 4 in the third octet.

Step 3 The first two new possible subnets (10.0.0.0/22, 10.0.4.0/22) overlap with existing subnets.

Step 4 10.0.8.0/22 is the numerically lowest new subnet number that does not overlap with the existing subnets.

Problem 4

The answer for this problem requires more detail than others, because the /25 mask creates a larger number of subnets that might overlap with the pre-existing subnets. For this problem, at Step 1, you already know to use mask /25. Table 5-13 shows the results of Step 2, listing the first 14 subnets of network 10.0.0.0 when using mask /25. For Step 4, Table 5-13 highlights the overlapped subnets. To complete the task at Step 5, search the table sequentially and find the first non-grayed subnet, 10.0.1.128/25.

Table 5-13. First 14 Subnets of Network 10.0.0.0, Using /25 Mask

Reference	Subnet Number	Broadcast Address
1	`10.0.0.0`	`10.0.0.127`
2	`10.0.0.128`	`10.0.0.255`
3	`10.0.1.0`	`10.0.1.127`
4	10.0.1.128	10.0.1.255
5	`10.0.2.0`	`10.0.2.127`
6	10.0.2.128	10.0.2.255
7	`10.0.3.0`	`10.0.3.127`
8	10.0.3.128	10.0.3.255
9	10.0.4.0	10.0.4.127
10	10.0.4.128	10.0.4.255
11	10.0.5.0	10.0.5.127
12	10.0.5.128	10.0.5.255
13	`10.0.6.0`	`10.0.6.127`
14	10.0.6.128	10.0.6.255

Problem 5

Like Problem 4, the answer for Problem 5 requires more detail, because the /26 mask creates a larger number of subnets that might overlap with the pre-existing subnets. For this problem, at Step 1, you already know to use mask /26. Table 5-14 shows the results of Step 2, listing the first 12 subnets of network 10.0.0.0 when using mask /26. For Step 4, Table 5-14 highlights the overlapped subnets. To complete the task at Step 5, search the table sequentially and find the first non-grayed subnet, 10.0.1.128/26.

Table 5-14. First 12 Subnets of Network 10.0.0.0, Using /26 Mask

Reference	Subnet Number	Broadcast Address
1	`10.0.0.0`	`10.0.0.63`
2	`10.0.0.64`	`10.0.0.127`
3	`10.0.0.128`	`10.0.0.191`
4	`10.0.0.192`	`10.0.0.255`
5	`10.0.1.0`	`10.0.1.63`
6	`10.0.1.64`	`10.0.1.127`
7	10.0.1.128	10.0.1.191
8	10.0.1.192	10.0.1.255
9	`10.0.2.0`	`10.0.2.63`
10	10.0.2.64	10.0.2.127
11	10.0.2.128	10.0.2.191
12	10.0.2.192	10.0.2.255

Answers to Practice Designing VLSM Subnets

This section lists the answers to the two practice problems in the section, “Practice Designing VLSM Subnets.”

Answers for VLSM Subnet Design, Problem 1

For Problem 1, subnetting network 172.20.0.0 with mask /22 means that the subnets will all be multiples of 4 in the third octet: 172.20.0.0, 172.20.4.0, 172.20.8.0, and so on, through 172.20.252.0. Following the rule to choose the numerically lowest subnet IDs, you would allocate or use 172.20.0.0/22, 172.20.4.0/22, and 172.20.8.0/22. You would also then mark the next subnet, 172.20.12.0/22, to be sub-subnetted.

For the next mask, /25, all the subnet IDs will be either 0 or 128 in the last octet, and increments of 1 in the third octet. Starting at 172.20.12.0 per the previous paragraph, the first four such subnets are 172.20.12.0/25, 172.20.12.128/25, 172.20.13.0/25, and 172.20.13.128/25. Of these, you need to use three, so mark the first three as used. The fourth will be sub-subnetted at the next step.

For the third and final mask, /30, all the subnet IDs will increment by 4 in the fourth octet. Starting with the subnet ID that will be sub-subnetted (172.20.13.128), the next /30 subnet IDs are 172.20.13.128, 172.20.13.132, 172.20.13.136, 172.20.13.140, and so on. The first three in this list will be the three used per the requirements and rules for Problem 1.

Answers for VLSM Subnet Design, Problem 2

For Problem 1, subnetting network 192.168.1.0 with mask /27 means that the subnets will all be multiples of 32 in the fourth octet: 192.168.1.0, 192.168.1.32, 192.168.1.64, 192.168.1.96, and so on, through 192.168.1.224. Following the rule to choose the numerically lowest subnet IDs, you would allocate or use 192.168.1.0/27, 192.168.1.32/27, and 192.168.1.64/27. You would also then mark the next subnet, 192.168.1.96/27, to be sub-subnetted.

For the next mask, /28, all the subnet IDs will be multiples of 16 in the last octet. Starting at 192.168.1.96 per the previous paragraph, the first four such subnets are 192.168.1.96, 192.168.1.112, 192.168.1.128, and 192.168.1.144. Of these, you need to use three, so mark the first three as used. The fourth will be sub-subnetted at the next step.

For the third and final mask, /30, all the subnet IDs will increment by 4 in the fourth octet. Starting with the subnet ID that will be sub-subnetted (192.168.1.144), the next /30 subnet IDs are 192.168.1.144, 192.168.1.148, 192.168.1.152, 192.168.1.156, and so on. The first three in this list will be the three used per the requirements and rules for Problem 1.

Return to Beginning Previous Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.

Privacy Notice

Overview

Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security

Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children

This site is not directed to children under the age of 13.

Marketing

Pearson may send or direct marketing communications to users, provided that

Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
Such marketing is consistent with applicable law and Pearson's legal obligations.
Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out

Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

As required by law.
With the consent of the individual (or their parent, if the individual is a minor)
In response to a subpoena, court order or legal process, to the extent permitted or required by law
To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
To investigate or address actual or suspected fraud or other illegal activities
To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links

This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020

Email Address