cidr

CIDR Calculation: A Step-by-Step Classroom Demonstration

CIDR (Classless Inter-Domain Routing) turns IP subnetting into simple arithmetic on bits. Follow the walk-through below to show your students exactly how to slice a /24 network into four /26 subnets and verify every key value along the way.

Overview

CIDR expresses a network as <base-IP>/<prefix-length>.

• prefix-length = number of fixed (network) bits.

• Remaining 32-prefix bits vary for hosts.

Example: 192.168.1.0/24

• 24 network bits → 8 host bits → 2⁸ = 256 total addresses → 251 usable (5 Ip's will be reserved reserved).

Demonstration Scenario

• Start with VPC‑style private block 192.168.1.0/24 (common classroom lab).

• Goal: create four equal-size subnets, each large enough for ≈ 60 hosts.

• Solution: borrow 2 more bits from the host portion.

  • New prefix = 24 + 2 = /26 → host bits = 6 → 2⁶ = 64 total, 62 usable per subnet.

Binary Logic Cheat-Sheet

Concept	Binary Operation	Result	Decimal Meaning
Network address	IP AND Netmask	All host bits 0	Identifies the subnet
Broadcast	IP OR (~Netmask)	All host bits 1	Last address in subnet
Usable range	Network + 1 → Broadcast – 1	—	Assignable to devices

Hands-On Walk-Through

1

Determine Subnet Mask

• /26 = first 26 bits are 1s → binary 11111111.11111111.11111111.11000000

• Convert to dotted-decimal → 255.255.255.192.

2

List the New Subnet Blocks

The original /24 ranges from 192.168.1.0 to 192.168.1.255. Increment size = 64 (2⁶) addresses.

Subnet #	CIDR	Network	First Usable	Last Usable	Broadcast	Total	Usable
1	192.168.1.0/26	192.168.1.0	192.168.1.1	192.168.1.62	192.168.1.63	64	62
2	192.168.1.64/26	192.168.1.64	192.168.1.65	192.168.1.126	192.168.1.127	64	62
3	192.168.1.128/26	192.168.1.128	192.168.1.129	192.168.1.190	192.168.1.191	64	62
4	192.168.1.192/26	192.168.1.192	192.168.1.193	192.168.1.254	192.168.1.255	64	62

(The table was generated live with Python using the ipaddress module.)

3

Visual Aid: Bit-Level View of Subnet #1

textNetwork bits (26)                 Host bits (6)
11111111.11111111.11111111.11 000000   = 192.168.1.0  (network)
                                 000001   = 192.168.1.1  (first host)
                                 ...
                                 111110   = 192.168.1.62 (last host)
                                 111111   = 192.168.1.63 (broadcast)

Teaching Tips

• Whiteboard the AND operation using 192.168.1.70 & 255.255.255.192 to show how the result collapses to 192.168.1.64 (proves which subnet the host belongs to).

• Mnemonic for host count: hosts = 2^(32-prefix) – 2 (subtract network & broadcast).

• Use online calculators (e.g., cidr.xyz) in class so students can verify their manual work instantly.

• Relate to AWS VPC limits: AWS allows subnet sizes /28–/16; five IPs per subnet are always reserved.

Practice Exercise for Students

1. Split 10.0.0.0/22 into eight equal subnets.

   • Ask: what is the new prefix? (/25)

   • How many usable IPs per subnet? (126)

2. Identify the broadcast address of 172.16.8.200/20.

3. Given 192.168.200.9/30, prove why it is not in the same subnet as 192.168.200.5/30.

Key Takeaways

• CIDR offers flexible, non-classful subnet sizing, conserving address space.

• Subnetting is pure binary math: decide hosts required → calculate prefix → enumerate ranges.

• Mastering CIDR is essential for cloud design (AWS, Azure, GCP), on-prem routing, firewalls, and ACLs.

Guide your class through these steps live, and they’ll grasp CIDR calculation with confidence.