Home > Articles > Cisco > CCNA Routing and Switching

This chapter is from the book

VLSM Addressing Schemes

Variable-length subnet masking (VLSM) subnetting is similar to traditional subnetting in that bits are borrowed to create subnets. The formulas to calculate the number of hosts per subnet, and the number of subnets created still apply. The difference is that subnetting is not a single-pass activity.

VLSM Review

You probably noticed that the starting address space in Subnetting Scenario 3 is not an entire classful address. In fact, it is subnet 5 from Subnetting Scenario 2. So in Subnetting Scenario 3, you “subnetted a subnet.” That is what VLSM is in a nutshell: subnetting a subnet.

Let’s use a small example. Given the address space 172.30.4.0/22 and the network requirements shown in Figure 9-1, apply an addressing scheme that conserves the most amount of addresses for future growth.

Figure 9-1

Figure 9-1 VLSM Example Topology

We need five subnets: four LAN subnets and one WAN subnet. Starting with the largest host requirement on LAN 3, begin subnetting the address space.

To satisfy the 250 hosts requirement, we leave 8 hosts bits (28 – 2 = 254 hosts per subnet). Because we have 10 host bits total, we borrow 2 bits to create the first round of subnets (22 = 4 subnets). The starting subnet mask is /22 or 255.255.252.0. We turn on the next two bits in the subnet mask to get /24 or 255.255.255.0. The multiplier is 1. The four subnets are as follows:

  • Subnet 0: 172.30.4.0/24
  • Subnet 1: 172.30.5.0/24
  • Subnet 2: 172.30.6.0/24
  • Subnet 3: 172.30.7.0/24

Assigning Subnet 0 to LAN 3, we are left with three /24 subnets. Continuing on to the next largest host requirement on LAN 4, we take Subnet 1, 172.30.5.0/24, and subnet it further.

To satisfy the 100 hosts requirement, we leave 7 bits (27 – 2 = 128 hosts per subnet). Because we have 8 host bits total, we can borrow only 1 bit to create the subnets (21 = 2 subnets). The starting subnet mask is /24 or 255.255.255.0. We turn on the next bit in the subnet mask to get /25 or 255.255.255.128. The multiplier is 128. The two subnets are as follows:

  • Subnet 0: 172.30.5.0/25
  • Subnet 1: 172.30.5.128/25

Assigning Subnet 0 to LAN 4, we are left with one /25 subnet and two /24 subnets. Continuing on to the next largest host requirement on LAN 1, we take Subnet 1, 172.30.5.128/25, and subnet it further.

To satisfy the 60 hosts requirement, we leave 6 bits (26 – 2 = 62 hosts per subnet). Because we have 7 host bits total, we borrow 1 bit to create the subnets (21 = 2 subnets). The starting subnet mask is /25 or 255.255.255.128. We turn on the next bit in the subnet mask to get /26 or 255.255.255.192. The multiplier is 64. The two subnets are as follows:

  • Subnet 0: 172.30.5.128/26
  • Subnet 1: 172.30.5.192/26

Assigning Subnet 0 to LAN 1, we are left with one /26 subnet and two /24 subnets. Finishing our LAN subnetting with LAN 2, we take Subnet 1, 172.30.5.192/26, and subnet it further.

To satisfy the 10 hosts requirement, we leave 4 bits (24 – 2 = 14 hosts per subnet). Because we have 6 host bits total, we borrow 2 bits to create the subnets (22 = 4 subnets). The starting subnet mask is /26 or 255.255.255.192. We turn on the next two bits in the subnet mask to get /28 or 255.255.255.240. The multiplier is 16. The four subnets are as follows:

  • Subnet 0: 172.30.5.192/28
  • Subnet 1: 172.30.5.208/28
  • Subnet 2: 172.30.5.224/28
  • Subnet 3: 172.30.5.240/28

Assigning Subnet 0 to LAN 2, we are left with three /28 subnets and two /24 subnets. To finalize our addressing scheme, we need to create a subnet only for the WAN link, which needs only two host addresses. We take Subnet 1, 172.30.5.208/28, and subnet it further.

To satisfy the two hosts requirement, we leave 2 bits (22 – 2 = 2 hosts per subnet). Because we have 4 host bits total, we borrow 2 bits to create the subnets (22 = 4 subnets). The starting subnet mask is /28 or 255.255.255.240. We turn on the next 2 bits in the subnet mask to get /30 or 255.255.255.252. The multiplier is 4. The four subnets are as follows:

  • Subnet 0: 172.30.5.208/30
  • Subnet 1: 172.30.5.212/30
  • Subnet 2: 172.30.5.216/30
  • Subnet 3: 172.30.5.220/30

We assign Subnet 0 to the WAN link. We are left with three /30 subnets, two /28 subnets, and two /24 subnets.

VLSM Addressing Design Exercises

In the following VLSM addressing design exercises, you apply your VLSM addressing skills to a three router topology. Each exercise is progressively more difficult than the last. There may be more than one correct answer in some situations. However, you should always practice good addressing design by assigning your subnets contiguously.

Exercise 1

Assume that 4 bits were borrowed from the host portion of 192.168.1.0/24. You are not using VLSM. Starting with Subnet 0, label Figure 9-2 contiguously with subnets. Start with the LAN on RTA and proceed clockwise.

Figure 9-2

Figure 9-2 Addressing Design Exercise 1 Topology: Subnets

How many total valid host addresses will be wasted on the WAN links?

Now come up with a better addressing scheme using VLSM. Start with the same 4 bits borrowed from the host portion of 192.168.1.0/24. Label each of the LANs with a subnet. Then subnet the next available subnet to provide WAN subnets without wasting any host addresses. Label Figure 9-3 with the subnets.

Figure 9-3

Figure 9-3 Addressing Design Exercise 1 Topology: VLSM Subnets

List the address space that is still available for future expansion.

The topology shown in Figure 9-4 has LAN subnets already assigned out of the 192.168.1.0/24 address space. Using VLSM, create and label the WANs with subnets from the remaining address space.

Figure 9-4

Figure 9-4 Addressing Design Exercise 1 Topology: WAN Subnets

List the address space that is still available for future expansion.

Exercise 2

Your address space is 192.168.1.192/26. Each LAN needs to support ten hosts. Use VLSM to create a contiguous IP addressing scheme. Label Figure 9-5 with your addressing scheme. Don’t forget the WAN links.

Figure 9-5

Figure 9-5 Addressing Design Exercise 2 Topology

List the address space that is still available for future expansion.

Exercise 3

Your address space is 192.168.6.0/23. The number of hosts needed for each LAN is shown in Figure 9-6. Use VLSM to create a contiguous IP addressing scheme. Label Figure 9-6 with your addressing scheme. Don’t forget the WAN links.

Figure 9-6

Figure 9-6 Addressing Design Exercise 3 Topology

List the address space that is still available for future expansion.

Exercise 4

Your address space is 10.10.96.0/21. The number of hosts needed for each LAN is shown in Figure 9-7. Use VLSM to create a contiguous IP addressing scheme. Label Figure 9-7 with your addressing scheme. Don’t forget the WAN links.

Figure 9-7

Figure 9-7 Addressing Design Exercise 4 Topology

List the address space that is still available for future expansion.

Pearson IT Certification Promotional Mailings & Special Offers

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

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Pearson IT Certification 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@informit.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 Adobe 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.pearsonitcertification.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