611 Subnetting and CIDR for IoT Networks

611.1 Learning Objectives

By the end of this chapter, you will be able to:

Interpret subnet masks: Convert between dotted-decimal and CIDR notation
Calculate network addresses: Use binary AND operations to determine network boundaries
Apply CIDR notation: Calculate host counts from prefix lengths
Design IoT subnets: Plan network segmentation for security and management
Use VLSM: Allocate variable-sized subnets for efficient address utilization

MVU: Minimum Viable Understanding

Core concept: A subnet mask divides an IP address into network portion (identifies the segment) and host portion (identifies devices within that segment).

Why it matters: Subnetting enables security isolation, broadcast control, and logical organization of thousands of IoT devices.

Key takeaway: Plan /24 subnets (254 hosts) per floor, building, or sensor type - always include 30% growth buffer.

611.2 Prerequisites

Before diving into this chapter, you should be familiar with:

IPv4 Addressing Fundamentals: Understanding of 32-bit address structure and binary representation
Networking Basics: Foundation in networking concepts

For Beginners: What is Subnetting?

Subnetting is dividing a large network into smaller segments. Think of it like organizing a company: instead of one huge department with 10,000 employees, you create smaller teams (engineering, sales, HR).

Each subnet gets a range of IP addresses. For example, 192.168.1.0/24 gives you 254 usable addresses (192.168.1.1 through 192.168.1.254), perfect for a small office or building floor.

Why subnet? - Security: Isolate cameras from HVAC from sensors - Performance: Reduce broadcast traffic in each segment - Management: “All cameras are in 192.168.10.x” simplifies troubleshooting

Term	Simple Explanation
Subnet Mask	Defines which part of IP is network vs device (255.255.255.0)
CIDR Notation	Slash notation for subnet size (/24 = 255.255.255.0)
Network Address	First address in subnet, identifies the segment
Broadcast Address	Last address in subnet, reaches all hosts

611.3 Understanding Subnet Masks

A subnet mask determines which portion of an IP address represents the network and which represents the host. The mask is a 32-bit number where consecutive 1s represent the network portion, and 0s represent the host portion.

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graph LR
    subgraph Example["192.168.10.45 / 24"]
        Network["Network Portion<br/>192.168.10<br/>24 bits = 1s"]
        Host["Host Portion<br/>45<br/>8 bits = 0s"]
    end

    subgraph Mask["Subnet Mask: 255.255.255.0"]
        M1["255<br/>11111111"]
        M2["255<br/>11111111"]
        M3["255<br/>11111111"]
        M4["0<br/>00000000"]
    end

    Network -.->|"Identifies<br/>subnet"| M1
    Host -.->|"Identifies<br/>device"| M4

    style Network fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style Host fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style M1 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style M2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style M3 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style M4 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 611.1: Subnet mask 255.255.255.0 dividing IP into network and host portions

{fig-alt=“Subnet mask diagram showing how 192.168.10.45/24 is divided into network portion (192.168.10, 24 bits of 1s) and host portion (45, 8 bits of 0s), with corresponding subnet mask 255.255.255.0”}

Alternative View: IoT Subnet Planning by Device Count

This variant shows subnet selection based on actual IoT deployment needs:

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graph TB
    subgraph SMALL["Small IoT (Home)"]
        S1["/28 = 14 hosts<br/>Smart home devices"]
        S2["/27 = 30 hosts<br/>Home + garage"]
        S3["/26 = 62 hosts<br/>Large home"]
    end

    subgraph MEDIUM["Medium IoT (Building)"]
        M1["/24 = 254 hosts<br/>Single floor"]
        M2["/23 = 510 hosts<br/>Small building"]
        M3["/22 = 1022 hosts<br/>Office building"]
    end

    subgraph LARGE["Large IoT (Campus/City)"]
        L1["/20 = 4,094 hosts<br/>Campus network"]
        L2["/16 = 65,534 hosts<br/>Smart city zone"]
        L3["/8 = 16.7M hosts<br/>Enterprise global"]
    end

    style S1 fill:#16A085,stroke:#2C3E50,color:#fff
    style S2 fill:#16A085,stroke:#2C3E50,color:#fff
    style S3 fill:#16A085,stroke:#2C3E50,color:#fff
    style M1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style M2 fill:#E67E22,stroke:#2C3E50,color:#fff
    style M3 fill:#E67E22,stroke:#2C3E50,color:#fff
    style L1 fill:#2C3E50,stroke:#16A085,color:#fff
    style L2 fill:#2C3E50,stroke:#16A085,color:#fff
    style L3 fill:#2C3E50,stroke:#16A085,color:#fff

Planning tip: Always allocate 50% more addresses than current needs to accommodate growth. A home with 20 sensors should use /26 (62 hosts), not /27 (30 hosts).

Alternative View: Subnet Mask Binary Visualization

This variant shows how the binary mask pattern determines network/host division:

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graph LR
    subgraph MASK24["/24 Mask (255.255.255.0)"]
        M24["11111111.11111111.11111111|00000000<br/>Network (24 bits) | Host (8 bits)"]
    end

    subgraph MASK26["/26 Mask (255.255.255.192)"]
        M26["11111111.11111111.11111111.11|000000<br/>Network (26 bits) | Host (6 bits)"]
    end

    subgraph MASK28["/28 Mask (255.255.255.240)"]
        M28["11111111.11111111.11111111.1111|0000<br/>Network (28 bits) | Host (4 bits)"]
    end

    MASK24 -->|"Borrow 2 bits"| MASK26
    MASK26 -->|"Borrow 2 bits"| MASK28

    R24["254 hosts"]
    R26["62 hosts"]
    R28["14 hosts"]

    M24 --> R24
    M26 --> R26
    M28 --> R28

    style M24 fill:#16A085,stroke:#2C3E50,color:#fff
    style M26 fill:#E67E22,stroke:#2C3E50,color:#fff
    style M28 fill:#2C3E50,stroke:#16A085,color:#fff
    style R24 fill:#16A085,stroke:#2C3E50,color:#fff
    style R26 fill:#E67E22,stroke:#2C3E50,color:#fff
    style R28 fill:#2C3E50,stroke:#16A085,color:#fff

Each borrowed bit doubles the number of subnets but halves the hosts per subnet.

Common Subnet Masks:

CIDR	Subnet Mask	Binary	Network Bits	Host Bits	Total Hosts	Usable Hosts
/8	255.0.0.0	11111111.00000000…	8	24	16,777,216	16,777,214
/16	255.255.0.0	11111111.11111111.0…	16	16	65,536	65,534
/24	255.255.255.0	11111111.11111111.11111111.0	24	8	256	254
/25	255.255.255.128	…11111111.10000000	25	7	128	126
/26	255.255.255.192	…11111111.11000000	26	6	64	62
/27	255.255.255.224	…11111111.11100000	27	5	32	30
/28	255.255.255.240	…11111111.11110000	28	4	16	14
/29	255.255.255.248	…11111111.11111000	29	3	8	6
/30	255.255.255.252	…11111111.11111100	30	2	4	2

Why “Usable Hosts” is Less Than Total

Two addresses in each subnet are reserved: - Network address (all host bits = 0): Identifies the subnet itself (e.g., 192.168.1.0) - Broadcast address (all host bits = 1): Sends to all hosts on subnet (e.g., 192.168.1.255 for /24)

A /24 subnet has 256 total addresses, but only 254 are assignable to devices.

Show code

{
  const container = document.getElementById('kc-addr-3');
  if (container && typeof InlineKnowledgeCheck !== 'undefined') {
    container.innerHTML = '';
    container.appendChild(InlineKnowledgeCheck.create({
      question: "A factory floor manager needs to deploy 180 IoT sensors across a manufacturing line. The network administrator allocates subnet 10.50.100.0/24. After deployment, technicians report they can only connect 170 sensors. What happened?",
      options: [
        {text: "The /24 subnet only provides 170 addresses due to IPv4 header overhead", correct: false, feedback: "Header overhead affects packet size, not the number of available addresses. A /24 subnet provides 256 total addresses."},
        {text: "Ten addresses were reserved for broadcast traffic and cannot be assigned to devices", correct: false, feedback: "Only one address (the highest: .255 for /24) is reserved for broadcast. Not ten addresses."},
        {text: "The subnet provides 254 usable addresses, but some IPs were already taken by infrastructure (gateway, DHCP server) leaving exactly 170 available for sensors", correct: true, feedback: "Correct! A /24 has 256 addresses, minus network (.0) and broadcast (.255) = 254 usable. If infrastructure devices (gateway, DHCP server, switches, access points) consumed 84 addresses, only 170 remain for sensors. Always audit existing IP usage before planning new deployments."},
        {text: "IoT sensors require two IP addresses each (one for data, one for management), so 180 sensors need 360 addresses", correct: false, feedback: "IoT sensors typically use a single IP address. Multiple addresses per device are unusual except in specialized redundant configurations."}
      ],
      difficulty: "medium",
      topic: "addressing"
    }));
  }
}

611.4 Calculating Network and Broadcast Addresses

Example: 192.168.10.45/26

Step 1: Convert subnet mask to binary - /26 = 255.255.255.192 = 11111111.11111111.11111111.11000000

Step 2: Calculate network address (AND operation)

IP:    192  .  168  .   10  .  00101101  (45)
Mask:  255  .  255  .  255  .  11000000  (192)
       ---     ---     ---     --------
Net:   192  .  168  .   10  .  00000000  (0)

Network Address: 192.168.10.0

Step 3: Calculate broadcast address (set all host bits to 1)

Network: 192.168.10.00000000
Broadcast: 192.168.10.00111111 = 192.168.10.63

Step 4: Determine usable range - First usable host: 192.168.10.1 - Last usable host: 192.168.10.62 - Total usable hosts: 62

611.5 CIDR Notation and Classless Addressing

CIDR (Classless Inter-Domain Routing) replaced the rigid class-based system with flexible prefix lengths, enabling efficient address allocation.

CIDR Format: network-address/prefix-length - Example: 192.168.1.0/24 - /24 means “first 24 bits are network, remaining 8 bits are host”

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graph TD
    CIDR["CIDR Notation Examples"]

    C24["192.168.1.0/24<br/>254 usable hosts<br/>Small network"]
    C26["192.168.1.0/26<br/>62 usable hosts<br/>Small subnet"]
    C22["10.0.0.0/22<br/>1,022 usable hosts<br/>Large subnet"]
    C30["10.0.0.0/30<br/>2 usable hosts<br/>Point-to-point link"]

    CIDR --> C24
    CIDR --> C26
    CIDR --> C22
    CIDR --> C30

    style CIDR fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style C24 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C26 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C22 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C30 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 611.2: CIDR prefix lengths from /24 to /30 with host counts and use cases

{fig-alt=“CIDR notation examples showing different prefix lengths: /24 (254 hosts, small), /26 (62 hosts, smaller), /22 (1022 hosts, large), and /30 (2 hosts, point-to-point)”}

611.5.1 CIDR Calculation Formula

Number of addresses = 2^(32 - prefix length)

Examples: - /24: 2^(32-24) = 2^8 = 256 addresses - /26: 2^(32-26) = 2^6 = 64 addresses - /30: 2^(32-30) = 2^2 = 4 addresses (2 usable, for point-to-point links)

611.5.2 Quick Reference: Common IoT Subnet Sizes

Prefix	Hosts	Usable	Typical IoT Use Case
/30	4	2	Point-to-point links (gateway to gateway)
/29	8	6	Very small sensor clusters
/28	16	14	Small room automation (lighting, HVAC)
/27	32	30	Single floor sensors and actuators
/26	64	62	Medium building floor, lab environment
/25	128	126	Large floor, small building
/24	256	254	Typical building network, standard choice
/23	512	510	Multi-building campus segment
/22	1024	1022	Large campus, industrial facility
/20	4096	4094	Smart city district

Show code

{
  const container = document.getElementById('kc-addr-4');
  if (container && typeof InlineKnowledgeCheck !== 'undefined') {
    container.innerHTML = '';
    container.appendChild(InlineKnowledgeCheck.create({
      question: "You are designing an IoT network for a hospital with 12 separate departments, each needing approximately 20 medical IoT devices (patient monitors, infusion pumps, etc.). The network team gives you 10.100.0.0/22. What is the most efficient subnet allocation?",
      options: [
        {text: "Allocate one /22 subnet shared by all departments - simpler management with 1022 available addresses", correct: false, feedback: "While technically possible, mixing all medical devices in one broadcast domain creates security risks (one compromised device can attack all others) and makes troubleshooting difficult. Regulatory compliance (HIPAA) often requires network segmentation by department."},
        {text: "Create twelve /27 subnets (30 usable hosts each), leaving room for department growth and future expansion", correct: true, feedback: "Correct! /27 provides 30 usable addresses per department (20 devices + 10 growth buffer). Twelve /27 subnets use only 384 of the 1024 addresses, leaving 640 addresses for future departments. This balances efficiency with security isolation."},
        {text: "Use twelve /24 subnets - standard practice for any department network", correct: false, feedback: "Twelve /24 subnets would require 12 × 256 = 3,072 addresses, but you only have 1,024 in a /22. Additionally, /24 is wasteful for just 20 devices (91% unused)."},
        {text: "Assign exactly 20 addresses per department using /28 subnets for maximum efficiency", correct: false, feedback: "/28 provides only 14 usable addresses (16 - 2 reserved), which cannot even accommodate the current 20 devices per department. Always round up to include growth headroom."}
      ],
      difficulty: "hard",
      topic: "addressing"
    }));
  }
}

611.6 Subnetting for IoT Networks

611.6.1 Why Subnet IoT Deployments?

1. Security Isolation - Separate IoT devices from corporate networks - Contain breaches (compromised camera can’t access HR data) - Apply different firewall rules per device type

2. Broadcast Domain Management - IoT devices often use mDNS, DHCP broadcasts - Large broadcast domains degrade performance - Subnetting reduces broadcast traffic

3. Logical Organization - Group devices by function, floor, building, or security level - Simplifies troubleshooting (“all cameras are in 192.168.10.x”) - Enables targeted firmware updates

4. QoS and Traffic Prioritization - Route critical sensor data (fire alarms) through priority paths - Throttle non-critical traffic (environmental sensors)

5. Scalability - Plan for growth (reserve address space for future devices) - Hierarchical addressing supports large deployments

611.6.2 IoT Subnetting Design Principles

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graph TD
    Network["192.168.0.0/16<br/>Smart Building Network"]

    Cameras["192.168.10.0/24<br/>Security Cameras<br/>(254 hosts)"]
    Sensors["192.168.20.0/23<br/>Temperature Sensors<br/>(510 hosts)"]
    Lights["192.168.30.0/22<br/>Smart Lighting<br/>(1,022 hosts)"]
    Access["192.168.40.0/25<br/>Access Control<br/>(126 hosts)"]
    Infra["192.168.50.0/28<br/>Infrastructure<br/>(14 hosts)"]

    Network --> Cameras
    Network --> Sensors
    Network --> Lights
    Network --> Access
    Network --> Infra

    style Network fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style Cameras fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Sensors fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Lights fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Access fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Infra fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 611.3: Smart building IoT subnets organized by device type with appropriate sizes

{fig-alt=“IoT subnetting design showing smart building network divided by device type: cameras (/24, 254 hosts), sensors (/23, 510 hosts), lighting (/22, 1022 hosts), access control (/25, 126 hosts), and infrastructure (/28, 14 hosts)”}

Design Checklist: - [ ] Inventory all device types and quantities - [ ] Add 20-30% buffer for growth - [ ] Choose subnet sizes that accommodate growth - [ ] Reserve subnets for future expansion - [ ] Document IP ranges in network diagram - [ ] Plan for DHCP ranges vs static assignments - [ ] Consider VLAN tagging if using managed switches

611.6.3 Practical Subnetting Example: Smart Building

Scenario: Design addressing for a 10-floor smart office building with: - 50 HVAC sensors per floor (500 total) - 200 LED lighting controllers per floor (2,000 total) - 30 security cameras per floor (300 total) - 20 access control readers per floor (200 total) - 10 environmental sensors per floor (100 total) - 5 network infrastructure devices per floor (50 total)

Total devices: ~3,150 devices

Solution 1: By Device Type (Flat Network)

Device Type	Count	Buffer	Total	Subnet	CIDR	Usable IPs	IP Range
HVAC Sensors	500	150	650	192.168.1.0	/22	1,022	.1.1 - .3.254
Lighting	2,000	600	2,600	192.168.4.0	/21	2,046	.4.1 - .11.254
Cameras	300	100	400	192.168.12.0	/23	510	.12.1 - .13.254
Access Control	200	60	260	192.168.14.0	/24	254	.14.1 - .14.254
Environmental	100	30	130	192.168.15.0	/24	254	.15.1 - .15.254
Infrastructure	50	15	65	192.168.16.0	/26	62	.16.1 - .16.62

Solution 2: By Floor (Hierarchical)

Each floor gets 10.X.0.0/16, where X = floor number: - Floor 1: 10.1.0.0/16 (subdivided into device types) - Floor 2: 10.2.0.0/16 - Floor 3-10: Similar pattern

Example Floor 1 subdivision:

Device Type	Subnet	CIDR	Usable IPs
HVAC	10.1.1.0	/26	62 (50 needed)
Lighting	10.1.2.0	/24	254 (200 needed)
Cameras	10.1.3.0	/27	30 (30 needed)
Access Control	10.1.4.0	/27	30 (20 needed)
Environmental	10.1.5.0	/28	14 (10 needed)
Infrastructure	10.1.6.0	/29	6 (5 needed)

Recommendation: Solution 2 (hierarchical by floor) offers better: - Scalability: Easy to add floors - Troubleshooting: “Problem on Floor 3” = check 10.3.x.x - Physical topology mapping: IP matches physical location - Firewall rules: Floor-level access controls

611.6.4 VLSM (Variable Length Subnet Masking)

VLSM allows different subnet sizes within the same network, maximizing address efficiency.

Example: Subdividing 192.168.10.0/24

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graph TD
    Parent["192.168.10.0/24<br/>256 total addresses"]

    S1["192.168.10.0/26<br/>Cameras (64)"]
    S2["192.168.10.64/26<br/>HVAC (64)"]
    S3["192.168.10.128/27<br/>Access Control (32)"]
    S4["192.168.10.160/27<br/>Environmental (32)"]
    S5["192.168.10.192/28<br/>Infrastructure (16)"]
    S6["192.168.10.208/28<br/>Future (16)"]
    S7["192.168.10.224/27<br/>Reserved (32)"]

    Parent --> S1
    Parent --> S2
    Parent --> S3
    Parent --> S4
    Parent --> S5
    Parent --> S6
    Parent --> S7

    style Parent fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style S1 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S2 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S3 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S4 fill:#E67E22,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S5 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S6 fill:#7F8C8D,stroke:#2C3E50,stroke-width:2px,color:#fff
    style S7 fill:#7F8C8D,stroke:#2C3E50,stroke-width:2px,color:#fff

Figure 611.4: VLSM subdividing /24 into variable-sized subnets for different IoT device types

{fig-alt=“VLSM example showing 192.168.10.0/24 subdivided into variable-sized subnets: two /26 (64 hosts each), two /27 (32 hosts each), two /28 (16 hosts each), with efficient use of all 256 addresses”}

Breakdown: 1. 192.168.10.0/26 (64 addresses) → Cameras 2. 192.168.10.64/26 (64 addresses) → HVAC 3. 192.168.10.128/27 (32 addresses) → Access Control 4. 192.168.10.160/27 (32 addresses) → Environmental 5. 192.168.10.192/28 (16 addresses) → Infrastructure 6. 192.168.10.208/28 (16 addresses) → Future expansion 7. 192.168.10.224/27 (32 addresses) → Reserved

Total: 256 addresses allocated with zero waste!

Tradeoff: Flat Network vs Hierarchical Subnetting for IoT

Option A: Flat network (single /16 or /8) - All 3,000+ devices in one broadcast domain. Simpler initial setup, no inter-subnet routing needed, DHCP serves all devices from one pool. Broadcast traffic: 3,000 devices x ARP requests + mDNS + DHCP = ~50-100 broadcast packets/second consuming 2-5% of 100 Mbps link.

Option B: Hierarchical subnetting (multiple /24-/28) - Devices grouped by type or location. Requires router/L3 switch ($500-2,000), more complex DHCP configuration, firewall rules per subnet. Broadcast traffic isolated: each subnet sees only 50-200 device broadcasts = ~5-10 packets/second per segment.

Decision Factors: Use flat for <500 devices, single building, and when all devices need to discover each other (mDNS/Bonjour). Use hierarchical when >500 devices (broadcast storms degrade Wi-Fi performance), security isolation needed (cameras separate from HVAC), or troubleshooting clarity matters (IP 10.1.x.x = Floor 1). Rule of thumb: segment when broadcast traffic exceeds 1% of link capacity.

Tradeoff: Static IP Assignment vs DHCP for IoT Devices

Option A: Static IP addresses - Each device configured manually with fixed IP, subnet mask, gateway. No DHCP dependency (works during network outages), predictable firewall rules, easy asset tracking. Management cost: 2-5 minutes per device setup, spreadsheet/IPAM database required, IP conflicts if mismanaged.

Option B: DHCP with reservations - Server assigns IPs automatically, reservations tie MAC address to specific IP. Self-healing (device reboots get correct IP), centralized management, automatic DNS registration. Dependencies: DHCP server must be available at boot, reservation database must be maintained, potential delays (DORA: 4 packets, 50-500ms).

Decision Factors: Use static for critical infrastructure (<20 devices), devices that must work during network failures (safety systems, gateways), or extremely constrained devices without DHCP client. Use DHCP for large deployments (>50 devices), devices with easy configuration interfaces, or when devices move between locations. Hybrid approach: static for gateways/servers, DHCP for sensors.

611.7 Summary

Subnet masks divide IP addresses into network and host portions using consecutive 1s (network) and 0s (host)
CIDR notation expresses masks compactly: /24 = 255.255.255.0 = 256 addresses (254 usable)
Network address is calculated by ANDing the IP with the mask; broadcast address has all host bits set to 1
Subnet sizing formula: 2^(32 - prefix length) = total addresses, minus 2 for network and broadcast
IoT subnetting provides security isolation, broadcast control, logical organization, and scalability
VLSM enables efficient address allocation with variable-sized subnets for different device types
Design best practice: Inventory devices, add 30% growth buffer, document everything, and consider hierarchical addressing for large deployments

611.8 What’s Next

Now that you understand subnetting and CIDR, continue with:

Ports and NAT: Learn how port numbers identify services and how NAT enables internet access
IPv6 for IoT: Explore the next-generation addressing with massive address space
DHCP and Address Resolution: Configure automatic IP assignment and understand ARP/ND protocols