Introduction
Network segmentation is crucial for security, performance, and efficient network management. However, misconfigurations in VLANs, subnets, or routing can cause connectivity issues, preventing devices from communicating properly.
This step-by-step guide will help you diagnose and fix network segmentation problems to ensure seamless communication across your IT infrastructure.
What Causes Network Segmentation Problems?
Network segmentation issues occur when devices in separate VLANs or subnets fail to communicate. Some common causes include:
✅ Misconfigured VLANs – Devices are assigned to the wrong VLAN or VLAN tagging is incorrect.
✅ Incorrect Subnet Settings – Issues with subnet masks, IP addressing, or DHCP scopes.
✅ Routing Errors – Missing or misconfigured inter-VLAN routing.
✅ Firewall Restrictions – Security rules blocking traffic between network segments.
✅ DHCP or DNS Issues – Devices fail to obtain an IP address or resolve hostnames.
Now, let’s go through the step-by-step troubleshooting process to resolve these issues.
Step 1: Identify the Network Segmentation Issue
How to Detect Segmentation Issues:
1️⃣ Check connectivity using Ping
Open Command Prompt (Windows) or Terminal (Mac/Linux) and ping another subnet:
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ping < destination IP >
- If there’s no response, the network is segmented.
2️⃣ Trace the network path
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tracert < destination IP > (Windows)
traceroute < destination IP > (Mac/Linux)
- If the request stops at a router or firewall, routing or security policies may be the issue.
3️⃣ Check VLAN Assignments
Log in to your switch and run:
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show vlan brief
- Ensure devices are in the correct VLAN.
If any of these tests fail, move to Step 2.
Step 2: Verify VLAN Configuration on Switches
A misconfigured VLAN is a common cause of network segmentation.
How to Check VLAN Assignments:
🔹 Log in to your switch (CLI or Web UI).
🔹 Run:
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show vlan brief
🔹 Ensure VLAN trunking is enabled:
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show interfaces trunk
How to Assign a Device to the Correct VLAN:
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interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
exit
🔹 Save changes and restart the switch.
If VLANs are configured correctly, move to Step 3.
Step 3: Ensure Proper Inter-VLAN Routing
When VLANs or subnets can’t communicate, routing between them is likely misconfigured.
How to Check Routing Configuration:
🔹 Run:
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show ip route
🔹 If a required route is missing, add it:
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ip route 192.168.2.0 255.255.255.0 192.168.1.1
🔹 Enable Inter-VLAN Routing:
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interface vlan 10
ip address 192.168.10.1 255.255.255.0
no shutdown
interface vlan 20
ip address 192.168.20.1 255.255.255.0
no shutdown
Ensure IP Routing is Enabled on Layer 3 Switches:
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ip routing
Restart your router/switch and test connectivity. If routing is properly configured but VLANs still can’t communicate, proceed to Step 4.
Step 4: Check Firewall Rules & ACLs
Firewalls and Access Control Lists (ACLs) might be blocking traffic between VLANs.
How to Verify Firewall Rules:
🔹 Log in to your firewall settings.
🔹 Check Traffic Rules for any deny rules preventing inter-VLAN communication.
🔹 If necessary, create an allow rule:
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access-list 101 permit ip 192.168.10.0 0.0.0.255 192.168.20.0 0.0.0.255
🔹 For pfSense/Ubiquiti Firewalls:
➝ Navigate to Firewall > Rules > VLANs and allow traffic between VLANs.
Save changes and restart the firewall.
If firewall settings are correct, proceed to Step 5.
Step 5: Verify DHCP & DNS Configuration
If some devices aren’t getting IP addresses, DHCP issues might be causing segmentation problems.
How to Check DHCP Configuration:
🔹 Run:
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show ip dhcp binding
🔹 Ensure a DHCP pool exists for each VLAN:
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ip dhcp pool VLAN10
network 192.168.10.0 255.255.255.0
default-router 192.168.10.1
dns-server 8.8.8.8
Test DNS Resolution:
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nslookup google.com
🔹 If DNS fails, manually configure it:
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ip name-server 8.8.8.8
If DHCP and DNS are configured correctly, move to Step 6.
Step 6: Monitor Network Traffic for Issues
If segmentation issues persist, use network monitoring tools to detect problems.
Recommended Network Monitoring Tools:
🔹 Wireshark – Analyze network packets.
🔹 PRTG Network Monitor – Check VLAN traffic.
🔹 SolarWinds – Detect routing & segmentation issues.
Check for Routing Loops:
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traceroute 192.168.20.1
🔹 If the route loops indefinitely, inspect static routes.
If network segmentation persists, move to Step 7.
Step 7: Restart Devices & Test Again
A reboot may be required for changes to take effect.
Final Steps:
✅ Restart routers, switches, and firewalls.
✅ Run a final ping test:
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ping 192.168.20.1
✅ If successful, network segmentation is resolved! 🎉
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