Resolve MAC Address Conflicts on Local Network

Problem overview

Your network is experiencing erratic behavior, random disconnects, and massive packet loss. This happens when two devices on the local network are broadcasting the same MAC (Media Access Control) address, confusing the router’s switching tables.

Common causes

• Cloning a MAC address on a router or PC and forgetting to revert it
• Copying virtual machines (Hyper-V, VMware) without regenerating their virtual MACs
• Misconfigured network bridges or bonding
• Spoofing software generating duplicate random addresses

Solution 1: Identify the conflict using ARP

1. Open Command Prompt as administrator on a machine experiencing the issue.
2. Run the following command to view the local ARP table:

   arp -a

3. Look through the list of physical addresses. If you see the same physical address assigned to two different IP addresses, you have found the conflict.
4. Note the IP addresses involved and track down which physical devices or virtual machines they belong to.

Solution 2: Change a virtual machine MAC address

1. If the conflict is caused by cloned VMs, shut down the offending virtual machine.
2. In Hyper-V: Right-click the VM -> Settings -> Expand the Network Adapter -> Advanced Features. Under MAC address, select Dynamic or manually type a new static address.
3. In VMware: Right-click the VM -> Settings -> Network Adapter -> Advanced. Click the Generate button next to the MAC address field.
4. Start the VM. It will now request a new IP and stop conflicting.

Solution 3: Revert MAC cloning on a router

1. Log into your router’s admin interface.
2. Navigate to the WAN or Internet Setup page.
3. Locate the MAC Address Clone setting.
4. Select Use Default MAC Address instead of using the cloned PC address.
5. Save settings and reboot the router.

Solution 4: Release DHCP leases

1. Once the duplicate MAC is fixed at the hardware/software level, the router might still have bad ARP cache data.
2. Open Command Prompt as admin on your PC and run:

   ipconfig /release
   arp -d *
   ipconfig /renew

3. Restart your router to clear its DHCP leases and ARP tables entirely, ensuring clean routing moving forward.