Subnetting and CIDR

[!NOTE] This module explores the core principles of Subnetting and CIDR, deriving solutions from first principles and hardware constraints to build world-class, production-ready expertise.

1. Why Subnet?

If we put 10,000 computers on one flat network, the Broadcast Traffic (like ARP) would overwhelm every device. Subnetting allows us to:

  1. Reduce Congestion: Limit broadcasts to a smaller group.
  2. Security: Isolate the “Finance” subnet from the “Public Wi-Fi” subnet.
  3. Efficiency: Stop wasting large blocks of IP addresses.

2. The Subnet Mask

A 32-bit number that “masks” the network portion of an IP address.

  • 1 bits = Network
  • 0 bits = Host
  • Example: 255.255.255.0 in binary is 11111111.11111111.11111111.00000000.

3. CIDR Notation (Classless Inter-Domain Routing)

Instead of writing the full mask, we use “Slash Notation” to count the number of 1s.

  • /24 = 255.255.255.0 (24 ones)
  • /16 = 255.255.0.0 (16 ones)
  • /8 = 255.0.0.0 (8 ones)

4. The Math of a Subnet

For any subnet, there are two “Reserved” addresses:

  1. Network ID: The first address. (All host bits are 0).
  2. Broadcast ID: The last address. (All host bits are 1).
  3. Usable Hosts: 2host bits - 2.

5. Interactive: Subnet Calculator

Watch the ranges change.

Input CIDR
192.168.1.0 /
Mask:
255.255.255.0
Total IPs:
256
Usable:
254
Broadcast:
192.168.1.255

6. Variable Length Subnet Masking (VLSM)

The process of dividing a network into subnets of different sizes based on the number of hosts required.

  • Example: You have 192.168.1.0/24.
  • Subnet A (50 hosts) → Needs /26.
  • Subnet B (20 hosts) → Needs /27.
  • Serial Link (2 hosts) → Needs /30. This avoids wasting IP addresses.