Instructor Note   The purpose of this target indicator is that the students recognize class A, B, and C IP addresses. Students should be able to classify IP addresses as A, B, or C. They should also be able to label the octets "network" and "host" as appropriate for that address class. Emphasize that the network numbers are assigned by an external agency; only the host numbers can be assigned locally. While you may have heard of other class-less IP addressing schemes (such as CIDR, classless interdomain routing), the concepts of A, B, and C addresses are still widely used. And many questions on the CCNA exam assume classful addressing. The "lab", a paper-based activity, requires approximately 30 minutes. This TI is related to CCNA Certification Exam Objectives #29, #30, and #36.

There are three classes of IP addresses that an organization can receive from the American Registry for Internet Numbers (ARIN) (or the organization's ISP). They are Class A, B, and C. ARIN now reserves Class A addresses for governments throughout the world (although a few large companies, such as Hewlett Packard, have received one in the past) and Class B addresses for medium-sized companies. All other requestors are issued Class C addresses. 

Lab Activity   In this lab, you will learn different classes of IP addresses and how TCP/IP networks operate.

Class A
When written in a binary format, the first (leftmost) bit of a Class A address is always 0. An example of a Class A IP address is 124.95.44.15. The first octet, 124, identifies the network number assigned by ARIN. The internal administrators of the network assign the remaining 24 bits. An easy way to recognize whether a device is part of a Class A network is to look at the first octet of its IP address, which will range from 0-126. (127 does start with a 0 bit, but has been reserved for special purposes.)

All Class A IP addresses use only the first 8 bits to identify the network part of the address. The remaining three octets can be used for the host portion of the address. Every network that uses a Class A IP address can have assigned up to 2 to-the-power of 24 (2²⁴) (minus 2), or 16,777,214, possible IP addresses to devices that are attached to its network.

Class B
The first 2 bits of a Class B address are always 10 (one and zero). An example of a Class B IP address is 151.10.13.28. The first two octets identify the network number assigned by ARIN. The internal administrators of the network assign the remaining 16 bits. An easy way to recognize whether a device is part of a Class B network is to look at the first octet of its IP address. Class B IP addresses always have values ranging from 128 to 191 in their first octet.

All Class B IP addresses use the first 16 bits to identify the network part of the address. The two remaining octets of the IP address can be used for the host portion of the address. Every network that uses a Class B IP address can have assigned up to 2 to-the-power of 16 (2¹⁶) (minus 2 again!), or 65,534, possible IP addresses to devices that are attached to its network.

Class C
The first 3 bits of a Class C address are always 110 (one, one and zero). An example of a Class C IP address is 201.110.213.28. The first three octets identify the network number assigned by ARIN. The internal administrators of the network assign the remaining 8 bits . An easy way to recognize whether a device is part of a Class C network is to look at the first octet of its IP address. Class C IP addresses always have values ranging from 192 to 223 in their first octet.

All Class C IP addresses use the first 24 bits to identify the network part of the address. Only the last octet of a Class C IP address can be used for the host portion of the address. Every network that uses a Class C IP address can have assigned up to 2⁸ (minus 2), or 254, possible IP addresses to devices that are attached to its network.