Classful addressing (Class A, B, C) in Computer Networks - Step-by-Step Execution

Concept Flow - Classful addressing (Class A, B, C)

Start: IP Address

↓

Check 1st Octet Value

↓

Class A

↓

Class B

↓

Class C

↓

Other Classes or Reserved

↓

Assign Network and Host Portions

↓

Use for Routing and Addressing

The IP address's first number decides its class (A, B, or C), which defines how the address is split between network and host parts.

Execution Sample

Computer Networks

IP = 130.25.10.5
Check first octet = 130
Class B: Network=130.25, Host=10.5

Determine the class of an IP by its first number and split network and host parts accordingly.

Analysis Table

Step	First Octet Value	Condition	Class Determined	Network Portion	Host Portion
1	130	Is 1 <= 130 <= 126?	No	-	-
2	130	Is 128 <= 130 <= 191?	Class B	130.25	10.5
3	-	Assign network and host portions based on Class B	-	130.25	10.5
4	-	End of classification	-	-	-

💡 First octet 130 falls in 128-191 range, so IP is Class B and split accordingly.

State Tracker

Variable	Start	After Step 1	After Step 2	Final
IP	130.25.10.5	130.25.10.5	130.25.10.5	130.25.10.5
First Octet	-	130	130	130
Class	-	-	Class B	Class B
Network Portion	-	-	130.25	130.25
Host Portion	-	-	10.5	10.5

Key Insights - 3 Insights

Why is 127 not considered Class A?

127 is reserved for loopback addresses and is not used as a normal Class A network. The execution_table shows the range check excludes 127.

How do we know which part of the IP is network and which is host?

The class determines the split: Class A uses first octet for network, Class B uses first two octets, Class C uses first three. The execution_table step 3 shows this split for Class B.

What happens if the first octet is 224 or higher?

Such addresses belong to Class D or E, used for multicast or experimental purposes, not covered in this basic classful addressing. The flow shows these go to 'Other Classes or Reserved'.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2: What class is assigned when the first octet is 130?

A Class B

B Class C

C Class A

D Class D

💡 Hint

Refer to the 'Class Determined' column at Step 2 in the execution_table.

According to variable_tracker, what is the network portion after Step 2?

A 130

B 10.5

C 130.25

D 25.10

💡 Hint

Check the 'Network Portion' row under 'After Step 2' in variable_tracker.

At which step does the IP address get classified as Class B?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Class Determined' column in execution_table to find when classification happens.

Concept Snapshot

Classful IP addressing divides IPs into classes by first octet:
Class A: 1-126 (Network.Host.Host.Host)
Class B: 128-191 (Network.Network.Host.Host)
Class C: 192-223 (Network.Network.Network.Host)
Used to separate network and host parts for routing.

Full Transcript

Classful addressing uses the first number of an IP address to decide its class. Class A covers 1 to 126, Class B covers 128 to 191, and Class C covers 192 to 223. Each class defines how many parts of the IP are network and how many are host. For example, Class B uses the first two numbers as network and the last two as host. This helps routers know which part identifies the network and which part identifies devices within that network. Addresses like 127 are reserved and not used for normal networks. Addresses above 223 belong to other classes not covered here.

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