IPFYI

IP Address Conflict Resolution

Beginner Connectivity

Two or more devices on the same network have been assigned the same IP address, causing both to experience intermittent connectivity failures. IP conflicts typically arise when a static IP assignment overlaps with the DHCP server's dynamic range, or when a DHCP server malfunctions and issues the same address twice.

Symptoms

  • ⚠ Windows shows a notification bubble: 'There is an IP address conflict with another system on the network'
  • ⚠ macOS shows: 'Another device on the network is using your computer's IP address'
  • ⚠ One or both affected devices lose network connectivity randomly, even when the other is idle
  • ⚠ ARP cache on the gateway shows the same IP mapped to two different MAC addresses
  • ⚠ Connectivity alternates between devices — works briefly then drops when the other device sends an ARP

Possible Root Causes

  • • A device was manually assigned a static IP that falls within the DHCP server's dynamic pool range
  • • DHCP server bug or misconfiguration issued the same lease to two clients simultaneously
  • • A new device was added to the network using a static IP previously reserved for another device
  • • DHCP server was offline when a device booted, causing the device to use a cached or self-assigned IP that later conflicts
  • • Virtual machines or containers sharing a host IP range and bridging to the physical network without MAC address isolation

Diagnosis Steps

Step 1: Identify the conflicting IP on your device

# Linux / macOS
ip addr show
# or
ifconfig

# Windows
ipconfig /all

Note the IP address shown. This is the address in conflict.

Step 2: Find the other device using the same IP

# Linux / macOS — send ARP request for the conflicting IP
arp -n 192.168.1.100   # replace with the conflicting IP

# Alternatively, use arping to detect both MAC addresses
arping -I eth0 -c 10 192.168.1.100
# If two different MAC addresses respond, you've confirmed the conflict

On Windows:

arp -a | findstr 192.168.1.100

Step 3: Check the DHCP server's lease table

Log in to your router admin panel and look for the DHCP client list or active lease table. Identify which MAC addresses have been assigned the conflicting IP.

Step 4: Scan the network for duplicate IPs

# Install arp-scan (Linux)
sudo apt install arp-scan
sudo arp-scan --localnet

# Look for duplicate IP entries with different MAC addresses in the output

Step 5: Determine if static IP is involved

# Check if this device has a manually configured static IP
# Linux
ip addr show | grep "inet "

# Windows
ipconfig /all | findstr "IPv4 Address"
# Static IPs show (Preferred) tag without DHCP lease info

Solution

Step 1: Release and renew the conflicting device's IP

On the device showing the conflict warning:

# Linux
sudo dhclient -r eth0   # release
sudo dhclient eth0      # request new lease

# macOS (via Network Preferences > Advanced > TCP/IP > Renew DHCP Lease)
# or via terminal:
sudo ipconfig set en0 DHCP

# Windows
ipconfig /release
ipconfig /renew

Step 2: Fix static IP assignments

If one device uses a static IP, move it outside the DHCP pool range:

  1. Log in to router admin panel
  2. Note the DHCP pool range (e.g., 192.168.1.100 to 192.168.1.200)
  3. Assign static IPs from outside this range (e.g., 192.168.1.10 to 192.168.1.50)
# Linux — set static IP outside DHCP pool
sudo ip addr add 192.168.1.20/24 dev eth0
sudo ip route add default via 192.168.1.1

Step 3: Use DHCP reservations instead of static IPs

Rather than configuring static IPs on devices directly, configure DHCP reservations (static DHCP) on the router. This ensures the DHCP server always assigns the same IP to a specific MAC address:

  1. In router admin, go to DHCP > DHCP Reservations
  2. Enter the device's MAC address and the desired IP
  3. The IP must be within the DHCP pool or reserved separately

Step 4: Flush ARP cache on affected devices

# Linux
sudo ip neigh flush all

# macOS
sudo arp -d -a

# Windows
netsh interface ip delete arpcache

Prevention

  • Always configure static IPs in a dedicated range outside your DHCP pool (e.g., DHCP pool: .100-.200, static range: .2-.50)
  • Use DHCP reservations on the router rather than static IPs configured on devices — the DHCP server becomes the single source of truth
  • Document all static and reserved IP assignments in a spreadsheet or IP address management (IPAM) tool
  • Enable duplicate address detection (DAD) on IPv6 networks and ARP probe validation on IPv4 — most modern OSes do this by default
  • Regularly audit the DHCP lease table for unexpected devices and remove stale leases

Related Protocols

ARP DHCP IPV4

Related Terms

ip-address arp dhcp subnet gateway

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