Can you Identify a Classful IP Address?

If you plan to take the Network+ exam, you should be able to identify a Classful IP address. Specifically, objective “1.3 Explain the purpose and properties of IP addressing” expects you to be able to identify Class A, B, C, and D addresses. This blog explains what you need to know about a Classful IP address for the Network+ exam and includes a couple of practice test questions to test your comprehension.

Example Classful IP Address Questions

Q1. Which of the following is the subnet mask for a Class A address?

A. 255.0.0.0

B. 255.255.0.0

C. 255.255.255.0

D. 255.255.255.255

Answer later in this blog.

Q2. The first three bits of an IP address are 110. What type of IP address is this?

A. Class A

B. Class B

C. Class C

D. Class D

Answer later in this blog.

Understanding Classful IP Addresses

IPv4 addresses were originally designed as Classful IP addresses. A Classful IP address has a predefined subnet mask based on the first number in the IP address. The three primary Classful IP addresses are Class A, Class B, and Class C and are shown in the following table with their subnet mask. Class D addresses are used for multicasting but do not have a predefined subnet mask.

As an example, the IP address 10.80.1.5 has a 10 as the first number. The number 10 is within the range of 0 to 127 so it is a Class A address. Because it is a Class A address, it has a subnet mask of 255.0.0.0.

Classful IP Address Challenge

You should be able to look at any valid IP address, determine what Class it is, and identify it’s subnet mask. Can you determine the Class and subnet mask of the following IP addresses?

• 172.16.33.9
• 10.80.3.5
• 192.168.1.5

Answer later in this blog.

Binary and Classful IP Addresses

Classful IP addresses can also be identified based on the most significant bits.

IPv4 uses 32 bits and is commonly expressed in dotted decimal format. For example, the IP address of 192.168.1.5 includes four decimals separated by dots, which is how it gets the ‘dotted decimal’ name. Each decimal represents eight binary bits and all 32 bits look like this: 1100000101010000000000100000101. If youre like most people, you can probably read the IP address in dotted decimal format easier than you can read the binary bits.

Each decimal in the IPv4 address represents eight bits. Four decimals with eight bits each gives you a 32-bit address. An ‘octet’ indicates eight and you’ll often hear each eight-bit section of an IP address called an octet, even when it is expressed as a decimal number.

When identifying a Classful IP address, you only need to look at the first decimal or the first octet.

The following table shows the binary and decimal values for eight bits. It also shows the value of the most significant bits (also called the leading bits) for Classful IP addresses. The ~ character indicates that the bit isn’t used to determine the Class.

*Note: Some platforms have trouble representing numbers in superscript format. Instead of using superscript, they use the caret (^) character. You might even see the caret (^) character on the exam instead of the exam expressing the value as superscript.

• Class A. The first bit is always a 0. With a 0 for 128, the highest value of the first decimal is 127.
• Class B. The first two bits are always 1 and 0. With a 1 for 128 and a 0 for 64, the minimum value is 128 and the highest value is 191.
• Class C. The first two bits are always 1 and 1. With a 1 for 128 and a 1 for 64, the minimum value is 192 (128 + 64) and the highest value is 223.
• Class D. The first four bits are always 1, 1, 1, and 0. With a 1 for 128, a 1 for 64, and a 1 for 32, the minimum value is 224 (128+ 64 + 32) and the highest value is 239.

Network+ Study Resources

If you’re studying for the Network+ exam, you might like to check out the following resources:

Exam Basics	Basic information about the Network+ exam on The Get Certified Get Ahead Main Site
Practice Test Questions	CompTIA Network+ N10-005 Practice Test Questions (Get Certified Get Ahead)This Kindle book includes 275 realistic practice test questions for the Network+ exam with full explanations so you’ll know why the correct answer is correct, and why the incorrect answers are incorrect. It also includes 175 flash cards to help you affirm the important concepts for this exam.
 Free Kindle Apps	You don’t have to own a Kindle to use any of the Kindle books. Amazon has created free Kindle apps for just about any platform. Download these free apps and you can then read any Kindle book.
Practice Test Questions	Practice test questions for your mobile device.The same 275 practice test questions and 175 flashcards are also available on just about any mobile device through LearnZapp.
Network+ Basics	CompTIA Network+ Basic Networking Components (A Get Certified Get Ahead Kindle Short).Get a solid understanding of the core networking components covered in the CompTIA Network+ N10-005 exam. Some common networking components include switches, routers, access points, and firewalls. They are used in LANs, MANs, WANs, and VPNs within a variety of different network topologies such as Star, Ring, Mesh, and Bus topologies. This Kindle short introduces these topics and includes topics to help network technicians successfully plan and implement a small office home office (SOHO) network.
Network+ OSI Model	CompTIA Network+ OSI Topics (A Kindle Short).Don’t let the OSI topics overwhelm you. This Kindle short covers the OSI and TCP/IP Model objectives for the Network+ exam without overwhelming you with the complexity of these models. It includes a full chapter on OSI and TCP/IP model topics and practice test questions to test your comprehension.
Network+ Tools	CompTIA Network+: Tools (A Get Certified Get Ahead Kindle Short).Learn the relevant topics related to command line tools, hardware tools, and software tools. This Kindle Short includes a full chapter on tools and 60 practice test questions to test your comprehension of these topics.
Network+ Wireless	CompTIA Network+ Wireless Topics (A Kindle Short).This Kindle short covers all of the wireless objectives for the Network+ exam. It includes a full chapter on Wireless topics and 55 practice test questions to test your comprehension of these topics.

Other resources:

• Network+ Blogs
• Information on Performance Based Questions

Classful IP Address Question Answers

Q1. Which of the following is the subnet mask for a Class A address?

A. 255.0.0.0

B. 255.255.0.0

C. 255.255.255.0

D. 255.255.255.255

Q1. A is correct. A subnet mask of 255.0.0.0 is used for Class A addresses. A subnet mask of 255.255.0.0 is used for Class B addresses. A subnet mask of 255.255.255.0 is used for Class C addresses. A subnet mask of 255.255.255.255 is not used for Class A, Class B, Class C, or Class D addresses.

Q2. The first three bits of an IP address are 110. What type of IP address is this?

A. Class A

B. Class B

C. Class C

D. Class D

Q2. C is correct. The first three bits of a Class C address are 110 resulting in an IP address range of 192.0.0.0 to 223.255.255.255. The first three bits of a Class A address are 000 resulting in an IP address range of 0.0.0.0 to 127.255.255.255. The first three bits of a Class B address are 010 resulting in an IP address range of 128.0.0.0 to 191.255.255.255. The first three bits of a Class D address are 111 resulting in an IP address range of 224.0.0.0 to 239.255.255.255.

Classful IP Address Challenge

You were given an IP address and needed to identify the Class and subnet mask. Here are the answers

• 172.16.33.9
  Class B, 255.255.0.0
• 10.80.3.5
  Class A, 255.0.0.0
• 192.168.1.5
  Class C, 255.255.255.0