Subnetting Practical

🧠 Subnetting Practical (Handwritten Style)

A complete step-by-step guide to solving subnetting questions — just the way interviews and exams expect.

Every type of problem is solved in detail so you can revise fast and effectively.

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🔹 Q1. How Many Subnets and Hosts Can Be Created?

Example: 192.168.1.0/26

Step-by-Step

1. Host bits = 32 - 26 = 6
2. Usable hosts = 2^6 - 2 = 62
3. Borrowed bits = /26 - /24 = 2 → Subnets = 2^2 = 4

✅ Answer: 4 subnets, 62 usable hosts per subnet

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🔹 Q2. Find Subnet Mask and Ranges for Prefix

Example: /27

Steps

1. Subnet mask = 255.255.255.224
2. Block size = 256 - 224 = 32
3. Ranges = 0–31, 32–63, 64–95, …

✅ Answer: Subnets increase by 32, each with 30 usable hosts

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🔹 Q3. Which Subnet Does an IP Belong To?

Example: IP = 192.168.1.70

Network = 192.168.1.0/27

Steps

1. Block size = 32
2. Ranges = 0–31, 32–63, 64–95
3. 70 lies in 64–95

✅ Answer: 192.168.1.64/27

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🔹 Q4. How Many Bits to Borrow?

Example: 10.0.0.0/24 → Need 16 subnets

Steps

1. 2^x ≥ 16 → x = 4
2. New prefix = /24 + 4 = /28
3. Usable hosts = 2^4 - 2 = 14

✅ Answer: Borrow 4 bits → /28 → 16 subnets, 14 usable hosts each

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🔹 Q5. Smallest Subnet for Given Hosts

Example: Need 50 hosts, starting from 172.16.0.0/24

Steps

1. 2^n - 2 ≥ 50 → n = 6
2. Host bits = 6 → Prefix = 32 - 6 = /26
3. Subnet mask = 255.255.255.192

✅ Answer: Use /26 (62 usable hosts)

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🔹 Q6. VLSM Allocation

Given: 192.168.10.0/24 → For 100, 50, 25, and 10 hosts

Steps

1. 100 → /25 → Range: 192.168.10.0 – 192.168.10.127
2. 50 → /26 → Range: 192.168.10.128 – 192.168.10.191
3. 25 → /27 → Range: 192.168.10.192 – 192.168.10.223
4. 10 → /28 → Range: 192.168.10.224 – 192.168.10.239

✅ Answer: Efficient VLSM allocation without overlaps

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🔹 Q7. Point-to-Point Links

• /30 → 4 total, 2 usable
  👉 Example: 10.1.1.0/30 → Hosts: 10.1.1.1, 10.1.1.2
• /31 → 2 total, 2 usable (RFC 3021)
  👉 Example: 10.1.1.4/31 → Hosts: 10.1.1.4, 10.1.1.5

✅ Answer: Use /31 to save IPs on point-to-point links

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🔹 Q8. Find Broadcast Address

Example: 192.168.5.96/27

Steps

1. Block size = 32
2. Subnet range = Starts at 96 → Next = 128
3. Broadcast = 128 - 1 = 127

✅ Answer: Broadcast = 192.168.5.127

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🔹 Q9. Supernetting (Combining Networks)

Supernetting = combining smaller networks into a larger one
🔑 Rule: Networks must be consecutive and aligned properly

Example 1:

Combine 192.168.0.0/24 + 192.168.1.0/24
→ New prefix = /23
→ Range = 192.168.0.0 – 192.168.1.255

✅ Answer: Supernet = 192.168.0.0/23

Example 2:

Combine 10.0.0.0/24 + 10.0.1.0/24
→ New prefix = /23
→ Range = 10.0.0.0 – 10.0.1.255

✅ Answer: Supernet = 10.0.0.0/23

Example 3:

Combine:

• 172.16.0.0/24
• 172.16.1.0/24
• 172.16.2.0/24
• 172.16.3.0/24

→ 4 networks = 2^2 = 4 → Borrow 2 bits → /22
→ Range = 172.16.0.0 – 172.16.3.255

✅ Answer: Supernet = 172.16.0.0/22

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🔹 Q10. Trick Cases to Know

• /32 → Single IP only (loopback), no hosts
• /31 → 2 usable hosts, no broadcast (RFC 3021)
• /30 → Classic point-to-point with 2 usable hosts

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📝 Practice Problems (With Solutions)

1. 192.168.2.0/24 → Create 8 Subnets

• Need 8 subnets → 2^3 = 8 → Borrow 3 bits → /27
• Block size = 256 - 224 = 32
• Ranges: 0–31, 32–63, …, 224–255
• Each subnet = 30 usable hosts

✅ Answer: 8 subnets of /27

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2. Broadcast of 10.0.4.0/22

• Mask = 255.255.252.0
• Block size = 4 (in 3rd octet)
• Next = 10.0.8.0 → Broadcast = 10.0.7.255

✅ Answer: 10.0.7.255

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3. Which Subnet for 172.16.35.200/20?

• Mask = 255.255.240.0 → Block size = 16 (3rd octet)
• Ranges: 0–15, 16–31, 32–47, …
• 35 → in range 32–47

✅ Answer: 172.16.32.0/20

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4. Aggregate 192.168.8.0/24 + 192.168.9.0/24

• Two consecutive /24 networks
• Combine → /23
• Range = 192.168.8.0 – 192.168.9.255

✅ Answer: 192.168.8.0/23

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5. Need 500 Hosts → Which Prefix?

• 2^n - 2 ≥ 500 → n = 9
• Host bits = 9 → Prefix = /23
• Mask = 255.255.254.0 → 510 usable

✅ Answer: Use /23 (510 usable hosts)

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📌 Essential Subnetting Formulas

Hosts per Subnet = 2^h – 2 (where h = host bits)
Number of Subnets = 2^n (where n = borrowed bits)
Block Size = 256 – Subnet Mask Value
New Prefix = Old Prefix + Borrowed Bits
Subnet Mask = Prefix → Decimal Mask
Broadcast Address = Last IP in Subnet
Network Address = First IP in Subnet
Supernetting Rule = Combine only contiguous networks with common prefix

Special Cases:

• /32 = single host (loopback)

• /31 = 2 usable (RFC 3021, point-to-point)

• /30 = 2 usable (classic point-to-point)

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