Subnetting becomes essential when broadcast traffic exceeds 20% of total network traffic. This is measurable using network analyzers like Wireshark:

# Wireshark filter to measure broadcast traffic
(broadcast) && !(arp || stp)

Other measurable triggers include:

• Sustained 60-70% bandwidth utilization during peak hours
• Latency spikes exceeding 150ms for local network requests
• More than 200 active hosts per collision domain

PCI-DSS compliance requires separate subnets for:

# Example network segmentation for PCI compliance
10.0.1.0/24 - Cardholder Data Environment (CDE)
10.0.2.0/24 - DMZ 
10.0.3.0/24 - Internal corporate

Security best practices dictate subnetting when:

• Implementing zero-trust architectures
• Separating IoT devices from critical infrastructure
• Creating isolated test environments

For high-performance computing clusters, subnet based on:

# InfiniBand subnetting example
# Separate subnets for storage, compute, and management
192.168.100.0/24 - Storage network
192.168.101.0/24 - MPI traffic
192.168.102.0/24 - Cluster management

Consider subnetting when:

• Managing more than 15 VLANs becomes cumbersome
• Route summarization would reduce routing tables by >30%
• Different departments require independent QoS policies

Python subnet calculator for automation scenarios:

import ipaddress

def calculate_subnets(base_network, needed_subnets):
    network = ipaddress.IPv4Network(base_network)
    prefix_increment = (needed_subnets-1).bit_length()
    new_prefix = network.prefixlen + prefix_increment
    if new_prefix > 30:
        raise ValueError("Too many subnets requested")
    return list(network.subnets(new_prefix=new_prefix))

# Example usage:
print(calculate_subnets('192.168.0.0/16', 8))

Subnetting becomes essential when a single broadcast domain grows too large, leading to inefficiencies in network performance, security, or management. Here are measurable triggers to consider:

• Broadcast Traffic Overload: When broadcast packets (e.g., ARP, DHCP) exceed 20% of total traffic, latency spikes.
• Security Segmentation: Isolate departments (e.g., HR, Finance) or IoT devices to limit breach exposure.
• IP Address Exhaustion: If >80% of addresses in a /24 (254 hosts) are used, plan for expansion.

// Example: Calculating subnet needs for a growing startup
const totalDevices = 300;  // Exceeds /24 capacity
const requiredSubnets = Math.ceil(totalDevices / 30);  // Aim for /27 (30 hosts/subnet)
console.log(Divide into ${requiredSubnets} /27 subnets);  // Output: "Divide into 10 /27 subnets"

Case Study: Splitting a corporate /22 network (1022 hosts) into departmental VLANs:

# Cisco IOS example
configure terminal
vlan 10
 name Engineering
vlan 20
 name Marketing
interface vlan 10
 ip address 192.168.1.1 255.255.255.0
interface vlan 20
 ip address 192.168.2.1 255.255.255.0

• Over-partitioning (subnets with <5 hosts waste addresses)
• Mismatched subnet masks causing routing black holes
• Forgetting to update ACLs after re-subnetting