🕸️ Day 4: Subnetting and CIDR Notation

Subnetting is like dividing a large pizza into slices so everyone gets their share without fighting. It's the art of efficient network management. 🍕

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🛣️ Network vs Router

Prior to moving towards subnetting and CIDR notation, here are some terms you must know.

A network is allocated to you by your ISP (think of it like a street). The router is just the gatekeeper of this street.

• Your ISP assigns you a public IP with a prefix that defines the range of that network.
  • Example: 192.168.10.0/24
  • /24 tells how many devices can exist in that network.
  • In this case, /24 means 256 addresses, 254 usable for devices.
• The router is the gateway, directing traffic between your devices and the outside world.

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🏠 Network Address

Every network has a network address, like the name of the street.

• Your ISP’s public IP belongs to a network.
• Example: 192.168.10.0/24 -> .0 is the Network Address.
• This is not assigned to a device, it simply identifies the network.

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📢 Broadcast Address

Imagine you want to send a wedding invitation to all houses on your street.

• Instead of knocking on every door, you take a megaphone and announce it to everyone at once.
• In networking, this megaphone is called the Broadcast Address.
• Broadcast is the last address in the network range and is used to send a message to all devices in that subnet.

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🔪 Subnetting

Subnetting is like dividing your network into smaller, private blocks.

• Analogy: You have 2 cars (seats for 4 each). Your neighbours want to join, but you don’t want everyone crammed into your car.
• You give them their own car. Both cars reach the same destination, but traffic is separated.
• Similarly, subnetting splits a network so devices are grouped separately, traffic is isolated, and management becomes easier.

Remember: Every subnet has its own Broadcast Address and Network Address.

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📏 CIDR Notation

Classless Inter-Domain Routing (CIDR) is a system where we use IP + Suffix to describe the size of a network.

For example, your ISP may assign you a Public IP + suffix combination, like 192.168.10.0/24 (/24 is common in home networks). By this /24, we can readily find the network size, the Network Address, and Broadcast Address.

🧮 How to Calculate?

Example 1: 192.168.10.0/24

1. Calculate Host Bits: Total bits in an IP = 32. Subtract the CIDR: 32 - 24 = 8.
2. Calculate Total IPs: 2^8 = 256. Range is 0-255.
3. Block Size: 256.
4. Network Address: The first IP -> 192.168.10.0.
5. Broadcast Address: The last IP -> 192.168.10.255.
6. Usable Hosts: 256 - 2 = 254 devices.

Note: The first 24 bits are for Network Identification and the remaining 8 bits are for Size.

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🧠 Tricky Example

Example 2: 192.168.50.70/26

1. Calculate Host Bits: 32 - 26 = 6.
2. Calculate Total IPs: 2^6 = 64.
3. Block Ranges:
   • 0 - 63
   • 64 - 127
   • 128 - 191
   • 192 - 255
4. Find the Range: Where does 70 fall? In the second range (64 - 127).
5. Results:
   • Network Address: 192.168.50.64
   • Broadcast Address: 192.168.50.127
   • Usable Hosts: 64 - 2 = 62

The step of finding the correct block is crucial! ⚠️

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