Day 63: Subnetting & CIDR Notation – The Backbone of IP Addressing (ECET 2026)

Concept Notes (Deep Explanation + Examples)

🔹 What is Subnetting?

Subnetting is the process of dividing a large IP network into smaller, more manageable sub-networks called subnets.

• Every network has an IP address and a subnet mask.
• Subnetting helps in efficient utilization of IP addresses, improves security, and reduces network congestion.

Example:
Suppose you have a network 192.168.1.0/24 (means 256 addresses).
You can divide it into four subnets of 64 addresses each using subnetting.

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🔹 Basics of IP Address & Subnet Mask

• IP Address: A unique identifier for each device on a network (like 192.168.1.10).
• Subnet Mask: Used to separate network part and host part of the IP.

Example:

• IP: 192.168.1.10
• Subnet Mask: 255.255.255.0 → means first 24 bits are network bits.

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🔹 Why Do We Subnet?

1. To avoid IP wastage.
2. To improve network performance.
3. To enhance network security by isolating groups.
4. To simplify routing using hierarchical addressing.

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🔹 Subnetting Formula

Number of subnets =
Number of hosts per subnet =

Where:

• n = number of borrowed bits from host part
• h = remaining host bits after subnetting

Example:
If we borrow 2 bits from the host part in a Class C network (/24 → /26):

• Subnets =
• Hosts =

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🔹 CIDR Notation (Classless Inter-Domain Routing)

CIDR is a method to represent IP addresses without relying on traditional classes (A, B, C).
It combines IP address + subnet mask bits like this:

Example:
192.168.10.0/24 → means 24 bits for network, 8 bits for hosts.

CIDR	Subnet Mask	Total Hosts
/24	255.255.255.0	254
/25	255.255.255.128	126
/26	255.255.255.192	62
/27	255.255.255.224	30
/28	255.255.255.240	14
/29	255.255.255.248	6
/30	255.255.255.252	2

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🔹 Real-World Example

In a college campus network:

• Admin divides the network 192.168.1.0/24 into 4 departments using subnetting.
  • Dept 1: 192.168.1.0/26
  • Dept 2: 192.168.1.64/26
  • Dept 3: 192.168.1.128/26
  • Dept 4: 192.168.1.192/26

Each department now has 62 usable IP addresses.

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🔹 Diagram (explained in words)

Imagine a rectangle (big network) labeled 192.168.1.0/24.
It’s divided into four smaller rectangles, each labeled /26.
Each smaller block has its own network ID, broadcast ID, and usable IP range.

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🔹 ECET-Level Tip

In ECET, subnetting questions mostly ask:

• Number of subnets
• Number of hosts
• Network & broadcast address
• CIDR representation

👉 Always remember to subtract 2 from total hosts (1 for network ID, 1 for broadcast ID).

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⚙️ Formulas (Plain LaTeX)

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🔟 10 MCQs (ECET + GATE Hybrid)

1. The subnet mask for a /27 network is:
   A) 255.255.255.224
   B) 255.255.255.240
   C) 255.255.255.192
   D) 255.255.255.128
2. How many usable hosts are there in a /29 network?
   A) 8
   B) 6
   C) 4
   D) 2
3. The CIDR notation 192.168.5.0/26 provides how many subnets from a Class C network?
   A) 2
   B) 4
   C) 8
   D) 16
4. In a subnet 192.168.1.64/26, the broadcast address is:
   A) 192.168.1.65
   B) 192.168.1.126
   C) 192.168.1.127
   D) 192.168.1.128
5. If 3 bits are borrowed from host part, how many subnets are created?
   A) 4
   B) 6
   C) 8
   D) 16
6. A /25 network provides how many usable host addresses?
   A) 128
   B) 126
   C) 64
   D) 62
7. CIDR helps in:
   A) Class-based addressing
   B) Reducing routing table size
   C) Increasing network delay
   D) Removing IP headers
8. Which of the following belongs to 10.0.0.0/8?
   A) 10.1.2.3
   B) 11.0.0.5
   C) 192.10.1.1
   D) 172.10.0.1
9. The default subnet mask of a Class B address is:
   A) 255.255.0.0
   B) 255.255.255.0
   C) 255.0.0.0
   D) 255.255.255.128
10. The number of host bits in a /20 network is:
    A) 12
    B) 8
    C) 16
    D) 20

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✅ Answer Key

Q No | Answer
1 | A
2 | B
3 | B
4 | C
5 | C
6 | B
7 | B
8 | A
9 | A
10 | A

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🧠 MCQ Explanations

1️⃣ /27 → 255.255.255.224, since 27 bits used for network, remaining 5 bits for hosts.
2️⃣ /29 → 8 total addresses, 6 usable (2 reserved).
3️⃣ Borrowing 2 bits → subnets.
4️⃣ /26 has 64 IPs → range 192.168.1.64–127 → broadcast is 127.
5️⃣ subnets when 3 bits borrowed.
6️⃣ /25 → 128 IPs → 126 usable.
7️⃣ CIDR = Classless → reduces routing entries.
8️⃣ Any address starting with 10 belongs to 10.0.0.0/8.
9️⃣ Class B default mask is 255.255.0.0.
10️⃣ /20 → 32−20=12 → 12 host bits.

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🎯 Motivation (ECET 2026 Specific)

Subnetting appears almost every year in ECET & GATE.
Once you master CIDR and subnet logic, you can solve 3–4 marks questions within seconds.
Be consistent — practice daily subnet conversions and IP calculations.
This topic directly boosts your Network & OS combined understanding — key for ECET 2026 toppers!

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📲 CTA (Fixed)

Join our ECET 2026 CSE WhatsApp Group for daily quizzes & study notes:
👉 https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9

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