Duplicate IP & DHCP Lease Conflict
1. Purpose
This document provides a detailed troubleshooting and resolution procedure for incidents where two or more devices on the same network segment are assigned the same IPv4 address.
Duplicate IP conflicts can cause intermittent connectivity issues and unpredictable network behavior, particularly in cloud-managed environments using Pronto devices and VLAN-segmented client networks.
These conflicts disrupt normal packet delivery and may impact critical services such as:
POS terminals
Network printers
VoIP phones
Client devices
In many cases, other parts of the network may continue functioning, which makes the issue harder to detect without proper troubleshooting.
2. Scope
This SOP applies to the following environments:
Supported Devices
PC61 LTE Routers
PC26 Wi-Fi Access Points
Pronto Switches
Network Deployments
Cloud-managed networks using the Pronto Controller
SMB environments such as:
Restaurants
Cafes
Retail stores
VLAN-segmented networks
Managed and unmanaged switches
This SOP does NOT apply to
❌ Full WAN / ISP outages
❌ ARP spoofing or malicious attacks
(These are covered under separate security incident procedures.)
3. Background
An IP address conflict occurs when two devices on the same Layer-2 network segment attempt to use the same IPv4 address.
Each device on a network must have a unique IP address to communicate properly.
When duplicate IP addresses occur:
ARP tables become inconsistent
Network devices cannot determine the correct destination MAC address
Packets may be delivered to the wrong device
Connectivity becomes intermittent or unstable
Common Causes
Duplicate IP conflicts usually occur due to:
A static IP address configured within the DHCP pool
Multiple DHCP servers issuing addresses from overlapping ranges
Unauthorized or rogue DHCP servers
Improper DHCP lease management
Misconfigured DHCP scope or exclusions
4. Problem Description
Duplicate IP conflicts can be observed from two perspectives:
Observed in Network Logs
Indicators seen in controller or network logs:
Clients intermittently fail to communicate
ARP table entries change frequently under Device tools section over PCC portal
Same IP mapped to multiple MAC addresses or endpoints in Client page device page under client list
Reported by Site / End Users
Customers may report symptoms such as:
POS terminals unexpectedly lose connectivity
Printers or VoIP phones become unreachable
Devices display network connectivity warnings
DHCP errors or IP conflict alerts over system
Multiple Clients showing with same IP address in Client page on PCC portal
Protocol Explanation
At the protocol level:
IPv4 requires one unique IP address per host
If two devices share the same address:
ARP resolution becomes unreliable, which causes Layer 2 frames forwarding issue on Switch and Router can wrongly forward packets based on duplicate ARP entries with different MACs
Devices may disconnect from the network or intermittent connectivity failure.
In severe cases, critical devices may become completely unreachable.
5. Business Impact
Duplicate IP conflicts can affect different services with varying levels of severity.
Impact Area | Severity |
POS Service | High |
Guest Wi-Fi | Medium |
Cloud Monitoring | Medium |
Client Networking | High |
If conflicts occur within the POS VLAN or business-critical network segments, the impact may immediately affect transaction processing and revenue operations.
6. Common Root Causes
A. Static IP Overlap
A device such as:
Printer
Security camera
Server
POS terminal
may be manually configured with a static IP address that falls within the DHCP pool range.
When the DHCP server later assigns the same IP to another device, a conflict occurs.
B. Multiple DHCP Servers
Two or more DHCP servers are active on the same network segment.
Example scenario:
PC61 router running DHCP
ISP modem/router or any rogue dhcp also running with same network range
Both servers issue addresses from overlapping ranges.
C. Rogue DHCP Server
An unauthorized device on the LAN begins offering DHCP leases.
Common sources include:
Consumer routers
Personal hotspots
Misconfigured IoT devices
Unauthorized network equipment
D. DHCP Lease Mismanagement
In some cases:
DHCP leases remain stale
Devices reconnect with old IP addresses
The DHCP server incorrectly reassigns previously used IPs
This can result in temporary address conflicts.
7. Typical Symptoms
Network Indicators
Network-level signs include:
Duplicate ARP replies for a single IP
DHCP logs showing multiple OFFER messages
Clients repeatedly disconnecting and reconnecting
Device-Level Indicators
Affected devices may show messages such as:
IP address conflict detected
Network connectivity warnings
Devices connecting but losing connectivity intermittently
These symptoms often appear when a device reconnects to the network or receives a new DHCP lease.
8. Detailed Troubleshooting Procedure
Step 1 — Identify Affected Devices
Action
Navigate to:
Pronto Controller → Clients List
Then:
Sort by IP address
Search for the suspected IP
Expected Output
Example:
IP Address | MAC Address | Client |
10.10.10.45 | AA:BB:CC:11:22:33 | POS Device |
10.10.10.45 | FF:EE:DD:44:55:66 | Guest Phone |
✔ If the same IP appears with two different MAC addresses, a conflict exists.
Step 2 — Inspect the ARP Table
Action
Check the ARP table on the PCC controller under Client ARP section, you may need to run multiple times the ARP scan
Example
10.10.10.45 → MAC AA:BB:CC:11:22:33
10.10.10.45 → MAC FF:EE:DD:44:55:66
✔ Multiple MAC addresses claiming the same IP confirms the issue.
Step 3 — Check for Static IP Assignments
Action
Inspect the configuration of affected devices.
Verify whether any device has a manually configured IP address.
Expected Behaviour
If a device has:
Static IP inside the DHCP pool
→ This will cause a conflict.
Corrective Action
Move the static IP outside the DHCP pool range.
9. Resolution Scenarios
Scenario A — Static IP Inside DHCP Pool
Action
Either:
Change the device’s static IP outside the DHCP pool
OR
Exclude the static IP from the DHCP scope by adding device mapping in DHCP or custom range to avoid static IP inclusion in DHCP pool range
Expected Result
No further conflict events for that address.
Scenario B — Multiple DHCP Servers
Action
Disable DHCP on the secondary router
Ensure PC61 is the only DHCP server for the VLAN
Expected Result
DHCP logs on client capture should show:
DHCP OFFER → PC61 only
No overlapping address assignments.
Scenario C — Rogue DHCP Server
Action
Identify the unauthorized DHCP device
Disconnect the device or disable its DHCP service
Expected Result
DHCP leases stabilize and conflicts stop.
Scenario D — DHCP Lease Mismanagement
Action
Clear or expire stale DHCP leases by rebooting Router (This will cause Connectivity Down)
Reboot affected client devices
Expected Result
Clients receive new unique IP addresses.
10. Validation After Resolution
After corrective action, confirm the issue is resolved by verifying:
No duplicate IP addresses in the client list.
ARP table shows one unique MAC per IP address.
DHCP logs show only one authoritative DHCP server.
Affected devices reconnect and operate normally.