When two distinct subnets share the same physical switch without VLAN separation, several technical complications arise that impact network performance, security, and management. Let's examine the practical consequences through the lens of an engineer troubleshooting real-world scenarios.

The most immediate issue is broadcast traffic propagation between subnets. Consider this ARP behavior example:

# tcpdump output showing cross-subnet ARP requests
18:42:15.123456 ARP, Request who-has 192.168.1.50 tell 192.168.1.1, length 46
18:42:15.123789 ARP, Request who-has 10.0.0.75 tell 10.0.0.1, length 46

All hosts receive broadcasts from both subnets, consuming bandwidth and CPU cycles unnecessarily. This becomes particularly problematic in large deployments.

Without VLAN isolation, Layer 2 attacks become trivial:

# Example ARP poisoning attempt between subnets
arpspoof -i eth0 -t 192.168.1.100 10.0.0.1

Attackers can intercept traffic between subnets since the switch forwards all frames regardless of IP addressing.

Hosts may attempt direct communication when they should route through a gateway. Observe this traceroute anomaly:

traceroute to 10.0.0.100 (10.0.0.100), 30 hops max
 1  192.168.1.254 (192.168.1.254)  1.023 ms
 2  * * *
 3  * * *

The packets never leave the switch, creating routing black holes that confuse troubleshooting.

Here's how to properly segment the subnets on a Cisco switch:

interface GigabitEthernet1/0/1
 switchport mode access
 switchport access vlan 10
!
interface GigabitEthernet1/0/2
 switchport mode access
 switchport access vlan 20
!
interface Vlan10
 ip address 192.168.1.1 255.255.255.0
!
interface Vlan20
 ip address 10.0.0.1 255.255.255.0

In a mixed-subnet environment, we typically observe:

• 35-50% higher collision rates
• 20% slower TCP throughput
• 2-3x more ARP timeouts

If VLANs aren't possible, consider these alternatives:

# Linux host-based firewall rules
iptables -A INPUT -s 10.0.0.0/24 -j DROP
iptables -A OUTPUT -d 10.0.0.0/24 -j DROP

While not ideal, this prevents accidental cross-subnet communication at the host level.

When multiple IP subnets coexist on the same broadcast domain (a single switch without VLANs), several network behaviors emerge that every network engineer should understand:

# Simple Python demonstration of ARP behavior
import scapy.all as scapy

def detect_cross_subnet_arp(interface):
    def packet_callback(packet):
        if packet.haslayer(scapy.ARP):
            src_ip = packet[scapy.ARP].psrc
            dst_ip = packet[scapy.ARP].pdst
            if (src_ip.split('.')[0:3] != dst_ip.split('.')[0:3]):
                print(f"Cross-subnet ARP detected: {src_ip} -> {dst_ip}")
    
    scapy.sniff(iface=interface, prn=packet_callback, store=0)

detect_cross_subnet_arp("eth0")

The most immediate impact is broadcast traffic propagation. Both subnets' broadcast packets (ARP, DHCP, etc.) will flood all ports:

• ARP requests from Subnet A will reach Subnet B hosts
• DHCPDISCOVER messages may be received by multiple DHCP servers
• Network storms in one subnet affect the other

Without VLAN separation, hosts can:

// C code snippet showing raw socket creation potential
#include 
#include 

int create_promiscuous_socket() {
    int s = socket(AF_INET, SOCK_RAW, IPPROTO_RAW);
    if(s == -1) {
        perror("Socket creation failed");
        return -1;
    }
    
    // Enable promiscuous mode
    struct ifreq ifr;
    strncpy(ifr.ifr_name, "eth0", IFNAMSIZ);
    if(ioctl(s, SIOCGIFFLAGS, &ifr) == -1) {
        perror("IOCTL failed");
        close(s);
        return -1;
    }
    
    ifr.ifr_flags |= IFF_PROMISC;
    if(ioctl(s, SIOCSIFFLAGS, &ifr) == -1) {
        perror("Promiscuous mode enable failed");
        close(s);
        return -1;
    }
    
    return s;
}

For inter-subnet communication, you'll need:

The combined broadcast domains create measurable impacts:

# Bash script to measure broadcast rates
#!/bin/bash

IFACE="eth0"
DURATION=60

echo "Monitoring broadcast packets on $IFACE for $DURATION seconds..."
tcpdump -i $IFACE -c 0 -w /tmp/bcast.pcap 'broadcast' &
sleep $DURATION
kill $!
pkt_count=$(tcpdump -r /tmp/bcast.pcap 2>/dev/null | wc -l)
echo "Broadcast packets per second: $((pkt_count / DURATION))"

When VLANs aren't an option, consider these approaches:

• Private VLANs (if switch supports them)
• Host-based firewalls (iptables/nftables rules)
• Port security features like MAC address limiting

Essential troubleshooting commands:

# Windows
arp -a
route print

# Linux
ip neigh
ip route show table all

# Cisco
show arp
show ip route