21 Addressing & Subnetting

Key Concepts

IP Addressing: The scheme for assigning unique identifiers to every network interface, enabling packet routing across networks
Hierarchical Addressing: Structuring IP addresses so that common high-order bits identify the network, enabling efficient aggregation of routes
Subnet Planning: The process of dividing an allocated IP address block into appropriately sized subnets for each network segment
Default Gateway: The router address that hosts forward packets to when the destination is not on the local subnet
Secondary Address: An additional IP address assigned to one interface, allowing it to serve two subnets — sometimes used for IoT-IT integration
DHCP Scope: The range of IP addresses a DHCP server is configured to offer to clients in a specific subnet
Address Exhaustion: Running out of available IP addresses in a subnet; forces renumbering or expansion

21.1 In 60 Seconds

Every networked IoT device needs a unique IP address, and subnetting divides large networks into smaller, manageable segments by splitting addresses into network and host portions. Proper addressing and subnetting are essential for security isolation, broadcast control, and organizing thousands of IoT devices across buildings or zones.

21.2 Overview

This comprehensive guide to IP addressing and subnetting has been organized into focused chapters for easier learning. Each chapter covers a specific aspect of network addressing essential for IoT deployments.

MVU: Minimum Viable Understanding

Core concept: Every device on a network needs a unique IP address, and subnets divide large networks into manageable segments using masks that separate the “network” from “host” portions of addresses.

Why it matters: Incorrect IP addressing causes silent failures – devices simply cannot communicate, with no helpful error messages to diagnose the problem.

Key takeaway: Use private IP ranges (10.x.x.x, 172.16-31.x.x, 192.168.x.x) for internal IoT networks, and plan /24 subnets (254 hosts) per floor, building, or sensor type.

How It Works: IP Addressing and Subnetting

Addressing and subnetting work together to organize devices on networks:

Step 1: Address Assignment – Each device receives a unique IP address (either via DHCP or static configuration)

Step 2: Subnet Mask Application – The subnet mask divides the IP into network and host portions (e.g., 192.168.1.100 with mask 255.255.255.0 means network=192.168.1.0, host=.100)

Step 3: Local vs Remote Decision – Device performs bitwise AND between destination IP and its subnet mask to determine if destination is on the same network

Step 4: Routing Decision – If destination is local (same network), use ARP to find MAC and send directly. If remote (different network), send to default gateway

Step 5: Gateway Forwarding – Router examines destination IP, consults routing table, and forwards packet toward destination network

Example: Device at 192.168.1.50/24 sends to 192.168.1.100 – same network (192.168.1.0), sends directly. Sends to 10.0.0.5 – different network, forwards to gateway at 192.168.1.1.

21.3 Chapter Guide

Select the chapter that matches your learning goals:

21.3.1 IPv4 Addressing Fundamentals

Topics: 32-bit address structure, binary conversion, address classes (historical), private IP ranges (RFC 1918), special-purpose addresses, MAC address OUI

Best for: Understanding the building blocks of IP addressing

21.3.2 Subnetting and CIDR

Topics: Subnet masks, network/host division, CIDR notation, calculating network and broadcast addresses, VLSM, IoT subnet design patterns

Best for: Learning to divide networks and plan efficient address allocation

21.3.3 Port Numbers and NAT

Topics: Port number ranges (well-known, registered, ephemeral), IoT protocol ports (MQTT, CoAP, HTTP), 5-tuple connections, NAT translation, NAT traversal patterns for IoT

Best for: Understanding service identification and internet connectivity for IoT

21.3.4 IPv6 for IoT

Topics: 128-bit addresses, compression rules, address types (global, link-local, unique-local), 6LoWPAN header compression, SLAAC, IPv4/IPv6 transition strategies

Best for: Preparing for next-generation IoT networks and Thread/Matter protocols

21.3.5 DHCP and Address Resolution

Topics: DHCP DORA process, lease management, DHCP options, DHCP reservations, ARP for IPv4, Neighbor Discovery for IPv6, troubleshooting IP conflicts

Best for: Configuring automatic IP assignment and understanding address resolution

21.4 Learning Objectives (Complete Guide)

By completing all chapters, you will be able to:

Assign IP Addresses: Configure IPv4 and IPv6 addresses for IoT devices and networks
Design Subnets: Calculate subnet masks and plan network segmentation for IoT deployments
Configure Port Numbers: Identify and use standard ports for IoT protocols (MQTT, CoAP, HTTP)
Implement NAT and DHCP: Set up address translation and automatic IP assignment for device networks
Plan Private Networks: Design internal networks using RFC 1918 private address ranges
Troubleshoot Addressing Issues: Diagnose IP conflicts, routing problems, and connectivity failures

For Beginners: Network Addressing and Subnetting

Every device on a network needs a unique address, just like every house needs a street address for mail delivery. Subnetting is the practice of dividing a large network into smaller sections, like dividing a city into neighborhoods. This makes it easier to manage traffic and keep things organized.

Sensor Squad: The Neighborhood Map!

“How does the network know where to send my data?” asked Sammy the Sensor. Max the Microcontroller drew a map. “Every device gets a unique IP address, like a house number. And subnetting divides the big network into neighborhoods so messages get delivered to the right area first.”

“Think of it like a school,” said Lila the LED. “The school is the network (10.0.0.0), each floor is a subnet (10.0.1.0 for floor one, 10.0.2.0 for floor two), and each classroom door has a number. If you want to find room 203, you go to floor 2 first, then find room 03.”

“In our smart building, we put all temperature sensors on one subnet, all motion detectors on another, and all cameras on a third,” Max continued. “That way, if the cameras send lots of video data, it does not clog up the subnet where our little temperature readings travel.”

Bella the Battery appreciated this. “Smaller subnets also mean fewer devices shouting broadcast messages to everyone, which saves energy. And if a security problem hits one subnet, the other subnets stay safe. Subnetting is like building walls between neighborhoods – good for traffic, good for security!”

21.5 Prerequisites

Before diving into these chapters, you should be familiar with:

Networking Basics: Foundation in networking concepts, IP addressing fundamentals, and protocol basics
Network Mechanisms: Understanding how datagrams, packet switching, and network communication work
Layered Network Models: Knowledge of where addressing operates within the OSI/TCP-IP models (primarily Layer 3 – Network Layer)

21.6 Quick Reference

21.6.1 Private IP Ranges (RFC 1918)

Range	CIDR	Addresses	Typical Use
10.0.0.0 – 10.255.255.255	/8	16,777,216	Large enterprise, smart cities
172.16.0.0 – 172.31.255.255	/12	1,048,576	Medium industrial networks
192.168.0.0 – 192.168.255.255	/16	65,536	Home automation, small buildings

21.6.2 Common IoT Ports

Protocol	Port	Transport	Use Case
MQTT	1883	TCP	Lightweight messaging
MQTT/TLS	8883	TCP	Secure MQTT
CoAP	5683	UDP	Constrained devices
HTTP	80	TCP	Web interfaces
HTTPS	443	TCP	Secure web/APIs

21.6.3 Subnet Quick Reference

CIDR	Usable Hosts	Use Case
/30	2	Point-to-point links
/28	14	Small room automation
/27	30	Single floor
/26	62	Medium building floor
/24	254	Standard building network
/22	1,022	Large campus

Check Your Understanding: Subnet Sizing

Try It: Subnet Calculator

Use the sliders below to explore how CIDR prefix length affects the number of usable host addresses, and how much of a /16 network block different subnets consume.

Show code

totalAddresses = Math.pow(2, 32 - prefix)
usableHosts = totalAddresses - 2
networkBits = prefix
hostBits = 32 - prefix

maskOctets = {
  let bits = "";
  for (let i = 0; i < 32; i++) {
    bits += i < prefix ? "1" : "0";
  }
  return [
    parseInt(bits.substring(0, 8), 2),
    parseInt(bits.substring(8, 16), 2),
    parseInt(bits.substring(16, 24), 2),
    parseInt(bits.substring(24, 32), 2)
  ];
}

Show code

html`<div style="background: #f8f9fa; border: 1px solid #dee2e6; border-radius: 6px; padding: 1.2em; font-family: Arial, sans-serif;">
  <div style="display: grid; grid-template-columns: 1fr 1fr; gap: 1em;">
    <div>
      <div style="font-weight: bold; color: #2C3E50; margin-bottom: 0.5em;">Subnet Details</div>
      <div><strong>CIDR Notation:</strong> /${prefix}</div>
      <div><strong>Subnet Mask:</strong> ${maskOctets.join(".")}</div>
      <div><strong>Network bits:</strong> ${networkBits}</div>
      <div><strong>Host bits:</strong> ${hostBits}</div>
    </div>
    <div>
      <div style="font-weight: bold; color: #16A085; margin-bottom: 0.5em;">Address Capacity</div>
      <div><strong>Total addresses:</strong> 2<sup>${hostBits}</sup> = ${totalAddresses.toLocaleString()}</div>
      <div><strong>Usable hosts:</strong> ${usableHosts.toLocaleString()}</div>
      <div><strong>Reserved:</strong> 2 (network + broadcast)</div>
    </div>
  </div>
  <div style="margin-top: 1em; padding-top: 0.8em; border-top: 1px solid #dee2e6;">
    <div style="font-weight: bold; color: #E67E22; margin-bottom: 0.5em;">IoT Sizing Guidance</div>
    <div>${usableHosts >= 1000 ? "Campus-scale: suitable for large multi-building deployments" :
           usableHosts >= 200 ? "Building-scale: good for a full building or large floor" :
           usableHosts >= 50 ? "Floor-scale: appropriate for a single floor or device group" :
           usableHosts >= 10 ? "Room-scale: fits a small cluster of IoT devices" :
           "Link-scale: point-to-point or very small device groups"}</div>
    <div style="margin-top: 0.5em;"><strong>Subnets in a /16 block:</strong> ${Math.pow(2, prefix - 16).toLocaleString()} subnets of /${prefix}</div>
  </div>
</div>`

21.7 Knowledge Check

Quiz: Addressing and Subnetting

Level 1: Beginner Example – Home Network Addressing

Scenario: Set up IP addresses for 5 smart home devices

Given: Router at 192.168.1.1, subnet mask 255.255.255.0 (/24)

Address Plan:

Smart thermostat: 192.168.1.10 (static, always at this address)
Security camera: 192.168.1.20 (static)
3 sensors: 192.168.1.100-102 (DHCP range)

Why This Works: Static IPs for devices you access directly (thermostat, camera), DHCP for sensors that just report data.

Level 2: Intermediate Example – Multi-Floor Building

Scenario: 3-floor building, 30 devices per floor, need isolation

Address Plan:

Floor 1: 10.1.1.0/24 (254 usable IPs, 30 devices + 224 buffer)
Floor 2: 10.1.2.0/24
Floor 3: 10.1.3.0/24

Benefits: Each floor is a separate subnet – broadcasts stay within the floor, firewall rules can isolate floors, and it is easy to identify which floor a device is on from its IP.

Level 3: Advanced Example – VLSM for Mixed Device Types

Scenario: Smart factory with different device densities

Given: 10.50.0.0/22 (1,022 usable addresses to allocate)

VLSM Allocation:

200 PLCs: 10.50.0.0/24 (254 usable addresses)
50 cameras: 10.50.1.0/26 (62 usable addresses)
30 access points: 10.50.1.64/27 (30 usable addresses)
10 servers: 10.50.1.96/28 (14 usable addresses)
Reserved for growth: 10.50.2.0/23 (510 usable addresses)

Why VLSM?: Different device types get appropriately sized subnets – no waste, efficient use of address space.

Concept Check: Subnetting for Security

21.8 Concept Relationships

Concept	Depends On	Enables	Common With
Subnetting	IP addresses, subnet masks	Network segmentation, security isolation	VLANs, routing
CIDR Notation	Binary representation	Efficient address allocation	VLSM, route aggregation
Private IP Ranges	RFC 1918 standard	Internal networks without public IPs	NAT, firewalls
Subnet Mask	Binary AND operation	Local vs remote routing decisions	Default gateway configuration
VLSM	Subnetting fundamentals	Variable-sized subnets	Efficient address utilization

Try It Yourself: Subnet Planning Exercise

Exercise: Plan subnets for a university campus IoT deployment

Given:

IP allocation: 10.100.0.0/16 (65,534 usable addresses)
5 buildings: Library, Science, Admin, Dorms, Sports Center
Devices per building: 50–300 IoT sensors, cameras, access points

Task 1: Calculate Building Needs

Library: 150 devices
Science: 300 devices (labs have many sensors)
Admin: 50 devices
Dorms: 200 devices
Sports: 100 devices

Task 2: Add 30% Growth Buffer

Library: 150 x 1.3 = 195 – need /24 (254 usable addresses)
Science: 300 x 1.3 = 390 – need /23 (510 usable addresses)
Admin: 50 x 1.3 = 65 – need /25 (126 usable addresses)
Dorms: 200 x 1.3 = 260 – need /23 (510 usable addresses)
Sports: 100 x 1.3 = 130 – need /24 (254 usable addresses)

Task 3: Assign Subnets (Largest First)

Science: 10.100.0.0/23 (390 devices, 510 available)
Dorms: 10.100.2.0/23 (260 devices, 510 available)
Library: 10.100.4.0/24 (195 devices, 254 available)
Sports: 10.100.5.0/24 (130 devices, 254 available)
Admin: 10.100.6.0/25 (65 devices, 126 available)

Putting Numbers to It

Calculating usable addresses requires accounting for network and broadcast addresses.

\(\text{Usable Addresses} = 2^{(32-\text{prefix})} - 2\)

Worked example for /24: \(2^{(32-24)} - 2 = 2^8 - 2 = 256 - 2 = 254\) usable addresses

For Dorms (260 devices needed):

/24 provides: 254 addresses (short by 6)
/23 provides: \(2^9 - 2 = 510\) addresses (fits with margin)
Cost of wrong sizing: 6 devices cannot connect, requiring emergency network redesign

Verification:

Total used: 510 + 510 + 254 + 254 + 126 = 1,654 addresses
Total available: 65,534
Utilization: 2.5% (plenty of room for expansion)

Common Mistake: Confusing /31 and /30 for Point-to-Point Links

The Mistake: “I only need 2 addresses for a point-to-point link between routers, so I will use a /32 subnet.”

Why It Fails: A /32 designates a single host address (\(2^0 = 1\)), not a subnet with two endpoints.

The Numbers:

/32: 1 address (single host, unusable as a subnet)
/31: 2 addresses (valid for point-to-point links per RFC 3021, widely supported by modern routers)
/30: 4 addresses, 2 usable (\(2^2 - 2 = 2\)) – the traditional choice

Traditional /30 approach (universally supported):

Network: 192.168.1.0 (unusable)
Router A: 192.168.1.1 (usable)
Router B: 192.168.1.2 (usable)
Broadcast: 192.168.1.3 (unusable)

Modern /31 approach (RFC 3021, saves address space):

Router A: 192.168.1.0
Router B: 192.168.1.1
No network or broadcast address reserved

Recommendation: Use /31 if your equipment supports RFC 3021 (most modern routers do). Otherwise, use /30. Never use /32 for a point-to-point link.

21.9 Practice Quizzes

Match: Addressing Concepts to Definitions

Order: Designing an IoT Subnet Plan

Place the following steps in the correct sequence for planning subnets for a new IoT building deployment.

Common Pitfalls

1. Planning Subnets Based on Current Device Count Only

IoT networks grow unpredictably. A subnet with 10 free addresses today may be exhausted in 6 months as more sensors are added. Fix: plan for 3–5× current capacity, using /22 or larger subnets for IoT VLANs that are expected to grow.

2. Putting All IoT Devices on the Same Subnet as IT Equipment

IoT devices are often less secure than IT equipment. A compromised sensor could attack servers on the same subnet. Fix: isolate IoT devices in a dedicated subnet/VLAN with firewall rules limiting their access to only the specific servers they need to reach.

3. Not Documenting the Addressing Scheme

Over time, addresses are assigned informally and the subnet fills unpredictably. Fix: maintain an IP Address Management (IPAM) spreadsheet or tool documenting every assigned address, device name, and MAC address.

21.10 What’s Next

After completing IP addressing and subnetting, continue with these related chapters:

Topic	Chapter	Description
Routing	Routing Fundamentals	Learn how routers use subnet information to forward packets between networks
Transport Layer	Transport Fundamentals	Understand TCP and UDP transport protocols that operate above the network layer
IoT Messaging	MQTT Fundamentals	Apply networking knowledge to IoT publish-subscribe messaging with port 1883
Networking Basics	Networking Basics	Review foundational networking concepts and topologies that underpin addressing
Network Models	Layered Network Models	Understand where IP addressing fits within the OSI and TCP/IP model layers
Hands-On Practice	Networking Labs and Quiz	Hands-on configuration exercises for subnetting and address planning

21.1 In 60 Seconds

21.2 Overview

21.3 Chapter Guide

21.3.1 IPv4 Addressing Fundamentals

21.3.2 Subnetting and CIDR

21.3.3 Port Numbers and NAT

21.3.4 IPv6 for IoT

21.3.5 DHCP and Address Resolution

21.4 Learning Objectives (Complete Guide)

21.5 Prerequisites

21.6 Quick Reference

21.6.1 Private IP Ranges (RFC 1918)

21.6.2 Common IoT Ports

21.6.3 Subnet Quick Reference

21.7 Knowledge Check

21.8 Concept Relationships

21.9 Practice Quizzes

Common Pitfalls

21.10 What’s Next

21.11 Related Chapters