Home > Articles

This chapter is from the book

Classless Interdomain Routing

Classless Interdomain Routing (CIDR) is referenced in RFCs 1518, 1519, and 2050, which were developed for Internet service providers (ISPs) so they could aggregate contiguous blocks for efficient addressing schemes. With the classful system of allocating IP addresses, anyone who needed more than 254 host addresses was often forced into a public Class B address, providing more than 65,500 host addresses. Many companies and organizations wasted even more addresses by using only a fraction of their 16 million host Class A addresses. As a matter of fact, only a small percentage of the allocated Class A and Class B address space has been actually assigned to host computers on the Internet.

It became clear that by circumventing the class system, you could actually allocate just the amount of address space needed. In a large network, you want to avoid the situation where routers are maintaining a large number of routes in the routing tables and the IP address pool is used up. The process of route summarization, also known as aggregation or supernetting, allows you to represent a string of network numbers is a single summary address. By implementing CIDR, an ISP can embody a block of addresses with a single summarized route or supernet.

For example, if you needed around 1,000 addresses, four Class C networks of 250+ hosts each could be supernetted to represent 1000 hosts with a single summarized address. Figure 3.1 shows four Class C routers with a 24-bit mask summarized at the ISP router with a 22-bit mask:

RouterA: (11000000.10101000.10000000.00000000)
Subnet mask: (11111111.11111111.11111111.00000000)
RouterB: (11000000.10101000.10000001.00000000)
Subnet mask: (11111111.11111111.11111111.00000000)
RouterC: (11000000.10101000.10000010.00000000)
Subnet mask: (11111111.11111111.11111111.00000000)
RouterD: (11000000.10101000.10000011.00000000)
Subnet mask: (11111111.11111111.11111111.00000000)

Figure 3.1Figure 3.1 Major ISPs use CIDR addressing schemes to summarize multiple Class C addresses.

In this example, the subnet includes all the addresses from to; therefore, by supernetting the subnet address to be 22 bits instead of 24 bits, it will look like this:

Supernet: (11000000.10101000.10000000.00000000)
Subnet mask: (11111111.11111111.11111100.00000000)
Broadcast: (11000000.10101000.10000011.11111111)

As seen in the binary version of the subnet mask, the network is represented by 22 bits and the host portion is made up of 10 bits, resulting in 210 – 2 = 1022. This offers the "around 1000" hosts that are needed in this scenario.

With the CIDR system, the subnet mask notation is expressed with a simplified shorthand. In the previous example, instead of writing the address and subnet mask as, subnet mask

the CIDR network address is simply written as

CIDR notation is also used with classful addresses and can be written as /8 for Class A, /16 for Class B, and /24 for Class C. With CIDR, the largest ISPs are given large portions of address space, typically with a subnet mask of /19 or smaller. The ISP's customers are then provided networks from the resulting large pool of addresses. As a conversion guide, Table 3.2 displays the CIDR conversions for standard Class B network and the various subnets masks. The table also includes the number of subnet bits, the subnet mask and CIDR mapping, the number of subnets, and the number of hosts per subnet.

Table 3.2 Class B-to-CIDR Conversion Table

Number of Bits

Subnet Mask


Number of Subnets

Number of Hosts per Subnet






















































It is imperative that you know how to express the various networks and subnets with their CIDR implementations.

IP Version 6

CIDR will most likely serve the Internet well for the next few years. However, waiting in the wings for its moment in the sun is the Internet Protocol version 6 (IPv6) with a 128-bit address. With IPv6, every person could receive one billion unique IP addresses.

The goals of IPv6 are to enhance the existing IPv4 protocol by providing much more address space, easier configuration, and more security for the next-generation Internet. ISPs, corporate enterprises, and government organizations are looking to IPv6 for help in supporting cutting-edge applications, such as IP telephony, and new networking services, as well as emerging IP-aware devices. The demands for increased address space and plug-and-play networking capabilities are answered by the implementation of IPv6. According to Cisco, the threefold benefits of IPv6 are the expansion of network addressing to service future growth in IP devices and users, the autoconfiguration capability of IP addressing, and integrated support for IP Security (IPSec) and mobile IP.

The extended IPv6 address goes from a 32-bit to a 128-bit addressing scheme with the colon (:) being used as a delimiter instead of the dot (.). IPv6 also introduces newer unicast and broadcasting methods, including anycast. Anycast is a communication link between a single sender and the single closest interface (in terms of routing distance) that is identified by the anycast address. An anycast address is assigned to multiple interfaces. To facilitate delivery, the routing system must know about the interfaces that have anycast addresses assigned to them and their distance as measured by routing metrics. A multicast address is implemented for one-to-many communications with transport to multiple interfaces. An anycast address, on the other hand, is used for one-to-one-of-many communication, with delivery to a single interface that represents a list of interfaces.

The IPv6 address is a hexadecimal-formatted, eight-part address, separated by colons. Because each part can equal a 16-bit number and is 8 parts long, the address length is 128 bits (16x8=128). IPv6 addresses are n:n:n:n:n:n:n:n, which equals a 4-digit hexadecimal integer. Here is an example IPv6 address:


IPv6 addresses can also be further simplified by suppressing the leading zeros within each 16-bit part. Realize that each block must have at least a single digit. Look at how the previous address can be further simplified in the following example:


Because some IPv6 addresses end up having long sets of zeros, you can further simplify the depiction of an IPv6 address by compressing the ad- dress to "::" (also known as a double colon). For example, the IPv6 ad- dress FE80:0:0:0:3BB:FF:ED9A:4CB3 could be compressed to FE80::3BB:FF:ED9A:4CB3. The multicast address FF0A:0:0:0:0:0:0:4 could be further compressed to FF0A::4. To determine how many 0 bits are expressed by the double colon, simply count the number of parts in the compressed address, subtract this number from 8, and then multiply the result by 16. For example, in the address FF0A::4, there are two parts (FF0A and 4.) The number of bits expressed by the double colon is then (8–2)x16=96. You can use zero compression only one time in a particular IPv6 address.

An IP version 4 interface normally has only one IPv4 address assigned to it (excluding subinterface configuration). IPv6 nodes, on the other hand, typically have multiple assigned IPv6 addresses (multihomed). These include the link-local address, additional unicast addresses, and the loopback address (::1). In addition, every IPv6 interface is listening for traffic on several standard multicast addresses as well. A link-local address enables a node to communicate with other nodes on the link and can be used to configure the node even further.

The link-local address is typically autoconfigured from a combination of the interface identifier and the link-local prefix FE80::0. The autoconfiguration feature enables nodes to connect to a network and automatically generate IPv6 addresses without needing manual configuration or a DHCP server. IPv6-enabled routers can advertise prefixes in router advertisement messages, as well as function as default routers for the link. Router advertisements are transmitted occasionally and in response to router solicitations sent by hosts at system startup. A node automatically configures an IPv6 address by simply appending its 64-bit interface ID to the 64-bit prefixes (see next paragraph) included in the router advertisements. These subsequent 128-bit IPv6 addresses generated by the node are then tested for duplicates to make sure that they are unique on the network. Hosts send router solicitation packets at system startup so that they can immediately autoconfigure without having to wait for the next scheduled router advertisement.

IPv6 prefixes for identifying subnets and routes are expressed in the same way as CIDR notation for IPv4. An IPv6 prefix is expressed in the address/prefix-length notation. For example, 24EB:D3::/48 is a route prefix and 24EB:D3:0:2F3B::/64 is a subnet prefix. The 64-bit prefix is used for discrete subnets and all subnets have a 64-bit prefix. Any prefix that is less than 64 bits is a summarized route or address range portion of the IPv6 address.

Using static routes in IPv6 is similar to configuring static routes for IPv4, with a few differences. The ipv6 route command is used to configure IPv6 static routes. Some of the IPv4 keywords, such as tag and permanent, are not yet supported. The syntax for a static Ipv6 command is

Router(config)# ipv6 route ipv6-prefix/prefix-length {ipv6-address | 
interface-type interface-number [ipv6-address]} [administrative-distance]

For example, the following is a sample static route that is being configured to a specified next hop address, with no interface:

Router(config)# ipv6 route 2001:0BD6::/422022:808B:F0FE::1

For detailed information on Cisco's efforts with IPv6, go to http://www.cisco.com/ipv6. The specifications for IPv6 can be found in RFC 1519 (http://www.faqs.org/rfcs/rfc1519.html). For the current list of permanently assigned IPv6 multicast addresses, visit http://www.iana.org/assignments/ipv6-multicast-addresses.

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.


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.


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.


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.


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


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


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.


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.


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