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🔢 Day 5: Multi-Subnetting and Subnet Masks

Networking Subnetting

Think of a Large Company with different departments: HR, Sales, and IT. Subnetting is how we give each department its own secure floor while keeping everyone in the same building. 🏢

🏗️ Multi-Subnetting

In Day 4, we learned how to find the usable host range, network address, and broadcast address for a single network. Today, we will scale up and find these values for multiple subnets within a larger network.

📜 The Golden Table

Before we dive in, you must memorize (or bookmark) this relationship between CIDR, hosts, and subnets:

CIDR Total IP Addresses Number of subnets
/24 256 1
/25 128 2
/26 64 4
/27 32 8
/28 16 16

Rule of Thumb: As you move down, the total number of IPs is halved, while the number of subnets doubles (1 → 2 → 4 → 8 → 16).

🧮 Example 1: Dividing into 4 Subnets

Goal: Divide 192.168.10.0/24 into 4 subnets.

🛠️ Solution:

  1. Identify New CIDR: Look at the table. To get 4 subnets, we need a /26 CIDR.
  2. Determine Host Size: A /26 network has 64 total hosts.
    • Total IPs: 64
    • Usable Hosts: 64 - 2 = 62 (excluding Network and Broadcast addresses).
  3. Create the Blocks: Start from zero and add blocks of 64:
    • 0, 64, 128, 192 (Next would be 256, which is out of range).
  4. The Subnet Table:
Range Network Address Broadcast Address Usable Range
0 - 63 192.168.10.0 192.168.10.63 192.168.10.1 to 192.168.10.62
64 - 127 192.168.10.64 192.168.10.127 192.168.10.65 to 192.168.10.126
128 - 191 192.168.10.128 192.168.10.191 192.168.10.129 to 192.168.10.190
192 - 255 192.168.10.192 192.168.10.255 192.168.10.193 to 192.168.10.254

🧮 Example 2: Dividing into 8 Subnets

Goal: Divide the same network into 8 subnets.

🛠️ Solution:

  1. Identify New CIDR: For 8 subnets, the table points to /27.
  2. Determine Host Size: A /27 CIDR provides 32 hosts per subnet.
  3. The Subnet Table:
Hosts Range Network Address Broadcast Address Usable Hosts
0 - 31 192.168.10.0 192.168.10.31 .1 to .30
32 - 63 192.168.10.32 192.168.10.63 .33 to .62
64 - 95 192.168.10.64 192.168.10.95 .65 to .94
96 - 127 192.168.10.96 192.168.10.127 .97 to .126
128 - 159 192.168.10.128 192.168.10.159 .129 to .158
160 - 191 192.168.10.160 192.168.10.191 .161 to .190
192 - 223 192.168.10.192 192.168.10.223 .193 to .222
224 - 255 192.168.10.224 192.168.10.255 .225 to .254

💡 Pro Tip: If a message is destined for a device in the same subnet, the sender "shouts" (broadcasts) it. If it’s for another subnet, it sends it to the Router for delivery.

🎭 Subnet Mask

A Subnet Mask is a 32-bit number used to distinguish the Network part from the Host part of an IP address.

Computers don't "see" IPs the way we do; they only understand binary. To make it human-friendly, engineers created Subnet Masks as an intermediary—think of it as a high-level language for networking.

🧮 Finding a Subnet Mask

To find a subnet mask, we count bits from left to right in descending order (starting with all 1s).

Example 1: Basic Mask

Find the Subnet Mask for 192.168.10.0/24.

  1. Think of the IP as a house with 4 rooms (octets). Each room has 8 switches (bits).
  2. Distribute the CIDR /24 into the rooms.
    • Room 1: 8 switches (All ON)
    • Room 2: 8 switches (All ON)
    • Room 3: 8 switches (All ON) -> Total 24.
    • Room 4: 8 switches (All OFF).
  3. Binary Form: 11111111.11111111.11111111.00000000
  4. Conversion: A room with all 1s equals 255. All 0s equals 0.
    • Result: 255.255.255.0

Example 2: Intermediate Mask

Find the Subnet Mask for 10.1.1.50/29.

  1. First 3 octets get 8 bits each (24 bits total, all 1s).
  2. The remaining 5 bits (29 - 24 = 5) go into the 4th octet.
  3. Switch Values: 128, 64, 32, 16, 8, 4, 2, 1.
  4. Turn ON the first 5 switches: 128 + 64 + 32 + 16 + 8 = 248.
  5. Result: 255.255.255.248

Example 3: Tricky Mask

Find the Subnet Mask for 172.16.0.0/20.

  1. First 2 octets are full (16 bits total).
  2. The remaining 4 bits (20 - 16 = 4) go into the 3rd octet.
  3. The 4th octet remains empty (0).
  4. Turn ON the first 4 switches in the 3rd octet: 128 + 64 + 32 + 16 = 240.
  5. Result: 255.255.240.0

The boring and confusing stuff finally ended! 😁🤣

← Day-04, Subnetting and CIDR QUOTES for Month-02 →