Classful addressing (Class A, B, C) in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Classful addressing in IP networks?

Classful addressing is an early method of dividing IP addresses into fixed classes (A, B, C, D, E) based on the first few bits, to organize networks by size and purpose.

beginner

What is the range of the first octet for Class A IP addresses?

Class A IP addresses have a first octet range from 1 to 126. They are designed for very large networks with many hosts.

intermediate

How many networks and hosts does Class B addressing support?

Class B supports 16,384 networks and up to 65,534 hosts per network, suitable for medium-sized networks.

beginner

What is the default subnet mask for Class C addresses?

The default subnet mask for Class C is 255.255.255.0, allowing up to 254 hosts per network.

intermediate

Why is Classful addressing considered inefficient today?

Classful addressing wastes IP addresses because fixed class sizes don't fit all network sizes well, leading to unused addresses and poor scalability.

Which class of IP address starts with a first octet between 192 and 223?

A Class C

B Class A

C Class B

D Class D

Class C addresses have their first octet in the range 192 to 223.

What is the default subnet mask for Class B addresses?

A 255.0.0.0

B 255.255.255.0

C 255.255.0.0

D 255.255.255.255

Class B uses the default subnet mask 255.255.0.0.

Which class is designed for the largest number of hosts per network?

A Class D

B Class A

C Class C

D Class B

Class A supports the largest number of hosts per network, up to over 16 million.

What is the main limitation of Classful addressing?

A It wastes IP addresses due to fixed class sizes

B It supports unlimited hosts

C It uses variable-length subnet masks

D It is used only for IPv6

Classful addressing wastes IP addresses because fixed class sizes don't fit all network sizes well.

Which class is typically used for small networks with fewer than 254 hosts?

A Class D

B Class B

C Class A

D Class C

Class C is used for small networks, supporting up to 254 hosts.

Explain the differences between Class A, B, and C IP addresses in terms of their range and typical use.

Think about how many hosts each class supports and their first octet ranges.

Why did the networking community move away from Classful addressing to Classless addressing?

Consider the problem of wasted IP addresses and how modern methods solve it.

Practice

1. Which class of IP address has the first octet range from 1 to 126?

easy

A. Class D

B. Class B

C. Class C

D. Class A

Solution

1. Step 1: Understand IP class ranges

   Class A addresses have the first octet from 1 to 126.

2. Step 2: Match the range to the class

   Since 1-126 matches Class A, this is the correct class.

3. Final Answer:

   Class A -> Option D

4. Quick Check:

   1-126 = Class A [OK]

Hint: Class A starts with 1-126 in first octet [OK]

Common Mistakes:

• Confusing Class A with Class B range
• Including 0 or 127 in Class A range
• Mixing Class D with Class A

2. Which of the following is the correct subnet mask for a Class B IP address?

easy

A. 255.0.0.0

B. 255.255.255.0

C. 255.255.0.0

D. 255.255.255.255

Solution

1. Step 1: Recall subnet masks for classes

   Class A uses 255.0.0.0, Class B uses 255.255.0.0, Class C uses 255.255.255.0.

2. Step 2: Identify Class B subnet mask

   The correct subnet mask for Class B is 255.255.0.0.

3. Final Answer:

   255.255.0.0 -> Option C

4. Quick Check:

   Class B mask = 255.255.0.0 [OK]

Hint: Class B mask has two 255 blocks at start [OK]

Common Mistakes:

• Choosing Class A or C subnet masks for Class B
• Confusing subnet mask with broadcast address
• Selecting 255.255.255.255 which is a host mask

3. What is the network class of the IP address 192.168.1.1?

medium

A. Class C

B. Class B

C. Class D

D. Class A

Solution

1. Step 1: Check the first octet of the IP

   The first octet is 192.

2. Step 2: Match first octet to class range

   Class C addresses range from 192 to 223, so 192.168.1.1 is Class C.

3. Final Answer:

   Class C -> Option A

4. Quick Check:

   192 in 192-223 = Class C [OK]

Hint: 192-223 first octet means Class C [OK]

Common Mistakes:

• Mistaking Class B range for Class C
• Confusing Class D (224-239) with Class C
• Ignoring the first octet range

4. Identify the error in this IP address classification: IP: 130.25.10.5 classified as Class C.

medium

A. 130.25.10.5 is Class B, not Class C

B. 130.25.10.5 is Class A, not Class C

C. 130.25.10.5 is Class C, classification is correct

D. 130.25.10.5 is Class D, not Class C

Solution

1. Step 1: Check the first octet of the IP

   The first octet is 130.

2. Step 2: Determine correct class range

   Class B ranges from 128 to 191, so 130.25.10.5 is Class B, not Class C.

3. Final Answer:

   130.25.10.5 is Class B, not Class C -> Option A

4. Quick Check:

   130 in 128-191 = Class B [OK]

Hint: 130 is in Class B range 128-191 [OK]

Common Mistakes:

• Classifying 130 as Class C (192-223)
• Confusing Class A and B ranges
• Ignoring first octet value

5. You have an IP address 10.0.0.1 and want to divide it into smaller networks. Which classful address and default subnet mask does it belong to, and how many hosts can each subnet support?

hard

A. Class B, 255.255.0.0, supports about 65,000 hosts

B. Class A, 255.0.0.0, supports about 16 million hosts

C. Class C, 255.255.255.0, supports about 254 hosts

D. Class D, no subnet mask, used for multicast

Solution

1. Step 1: Identify the class of IP 10.0.0.1

   The first octet is 10, which falls in Class A (1-126).

2. Step 2: Recall default subnet mask and host count for Class A

   Class A default mask is 255.0.0.0, allowing 2^24 - 2 = 16,777,214 hosts approximately.

3. Final Answer:

   Class A, 255.0.0.0, supports about 16 million hosts -> Option B

4. Quick Check:

   10 in 1-126 = Class A, hosts ≈ 16 million [OK]

Hint: Class A IPs start 1-126, huge host capacity [OK]

Common Mistakes:

• Confusing Class A with Class B or C
• Using wrong subnet mask for Class A
• Miscounting number of hosts