Home > Articles > Cisco > CCNA Routing and Switching

Cisco CCENT Practice and Study Guide: Subnetting IP Networks

This chapter reviews the process of subnetting IP networks. First, by reviewing a process for subnetting IPv4 networks, then, by practicing subnetting skills, including several scenarios.
This chapter is from the book

Understanding the hierarchical structure of the IP address and how to modify that hierarchy in order to more efficiently meet routing requirements is an important part of planning an IP addressing scheme. This chapter reviews the process of subnetting IP networks. First, we review a process for subnetting IPv4 networks. Then, you practice subnetting skills, including several scenarios. Then, we briefly review subnetting IPv6 networks.

Subnetting an IPv4 Network

The process of segmenting a network, by dividing it into multiple smaller network spaces, is called subnetting. These subnetworks are called subnets. Although subnetting calculators are plentiful and freely accessible on the Internet, you must know how to subnet without using a calculator when you sit for the CCENT exam. Furthermore, subnetting skill will serve you well when troubleshooting common IP addressing issues.

Subnetting in Four Steps

Everyone has a preferred method of subnetting. Each teacher will use a slightly different strategy to help students master this crucial skill. The method I prefer can be broken down into four steps:

Step 1. Determine how many bits to borrow based on the network requirements.

Step 2. Determine the new subnet mask.

Step 3. Determine the subnet multiplier.

Step 4. List the subnets, including subnetwork address, host range, and broadcast address.

Subnetting Example

The best way to demonstrate the four steps of subnetting is to use an example. Let’s assume that you are given the network address 192.168.1.0/24, you need 30 hosts per network, and want to create as many subnets as possible.

Determine How Many Bits to Borrow

Because our requirement specifies 30 host addresses per subnet, we need to first determine the minimum number of host bits to leave. The remaining bits can be borrowed:

Host Bits = Bits Borrowed + Bits Left

To provide enough address space for 30 hosts, we need to leave 5 bits. Use the following formula:

2BL – 2 = number of host addresses

where the exponent BL is bits left in the host portion.

Remember, the “minus 2” is to account for the network and broadcast addresses that cannot be assigned to hosts.

In this example, leaving 5 bits in the host portion will provide the right number of host address:

25 – 2 = 30

Because we have 3 bits remaining in the original host portion, we borrow all these bits to satisfy the requirement to “create as many subnets as possible.” To determine how many subnets we can create, use the following formula:

2BB = Number of subnets

where the exponent BB is bits borrowed from the host portion.

In this example, borrowing 3 bits from the host portion will create 8 subnets: 23 = 8.

As shown in Table 9-1, the 3 bits are borrowed from the far-left bits in the host portion. The highlighted bits in the table show all possible combinations of manipulating the 8 bits borrowed to create the subnets.

Table 9-1 Binary and Decimal Value of the Subnetted Octet

Subnet Number

Last Octet Binary Value

Last Octet Decimal Value

0

00000000

.0

1

00100000

.32

2

01000000

.64

3

01100000

.96

4

10000000

.128

5

10100000

.160

6

11000000

.192

7

11100000

.224

Determine the New Subnet Mask

Notice in Table 9-1 that the network bits now include the 3 borrowed host bits in the last octet. Add these 3 bits to the 24 bits in the original subnet mask and you have a new subnet mask, /27. In decimal format, you turn on the 128, 64, and 32 bits in the last octet for a value of 224. So, the new subnet mask is 255.255.255.224.

Determine the Subnet Multiplier

Notice in Table 9-1 that the last octet decimal value increments by 32 with each subnet number. The number 32 is the subnet multiplier. You can quickly find the subnet multiplier using one of two methods:

  • Method 1: Subtract the last nonzero octet of the subnet mask from 256. In this example, the last nonzero octet is 224. So, the subnet multiplier is 256 – 224 = 32.
  • Method 2: The decimal value of the last bit borrowed is the subnet multiplier. In this example, we borrowed the 128 bit, the 64 bit, and the 32 bit. The 32 bit is the last bit we borrowed and is, therefore, the subnet multiplier.

By using the subnet multiplier, you no longer have to convert binary subnet bits to decimal.

List the Subnets, Host Ranges, and Broadcast Addresses

Listing the subnets, host ranges, and broadcast addresses helps you see the flow of addresses within one address space. Table 9-2 documents our subnet addressing scheme for the 192.168.1.0/24 address space. Fill in any missing information.

Table 9-2 Subnet Addressing Scheme for 192.168.1.0/24: 30 Hosts Per Subnet

Subnet Number

Subnet Address

Host Range

Broadcast Address

0

192.168.1.0

192.168.1.1–192.168.1.30

192.168.1.31

1

192.168.1.32

192.168.1.33–192.168.1.62

2

192.168.1.65–192.168.1.94

192.168.1.95

3

192.168.1.96

4

192.168.1.128

192.168.1.129–192.168.1.158

192.168.1.159

5

192.168.1.161–192.168.1.190

192.168.1.191

6

192.168.1.192

7

Use the four subnetting steps to complete the following scenarios.

Subnetting Scenario 1

Subnet the address space 10.10.0.0/16 to provide at least 100 host addresses per subnet while creating as many subnets as possible.

  1. How many bits should your borrow?
  2. What is the new subnet mask in dotted-decimal and prefix notation?

  3. What is the subnet multiplier?

In Table 9-3, list the first three subnets, host ranges, and broadcast addresses.

Table 9-3 Subnet Addressing Scheme for Scenario 1

Subnet Number

Subnet Address

Host Range

Broadcast Address

0

1

2

Subnetting Scenario 2

Subnet the address space 10.10.0.0/16 to provide at least 500 subnet addresses.

  1. How many bits should your borrow?
  2. What is the new subnet mask in dotted-decimal and prefix notation?

  3. What is the subnet multiplier?

In Table 9-4, list the first three subnets, host ranges, and broadcast addresses.

Table 9-4 Subnet Addressing Scheme for Scenario 2

Subnet Number

Subnet Address

Host Range

Broadcast Address

0

1

2

Subnetting Scenario 3

Subnet the address space 10.10.10.0/23 to provide at least 60 host addresses per subnet while creating as many subnets as possible.

  1. How many bits should your borrow?
  2. What is the new subnet mask in dotted-decimal and prefix notation?

  3. What is the subnet multiplier?

In Table 9-5, list the first three subnets, host ranges, and broadcast addresses.

Table 9-5 Subnet Addressing Scheme for Example 3

Subnet Number

Subnet Address

Host Range

Broadcast Address

0

1

2

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