COMMAND
arpView and edit the ARP cache
arp shows and manages the ARP cache, the table that maps local IP addresses to their MAC (hardware) addresses. arp -a lists the cache, arp -d * clears it, and inspecting it helps diagnose local-network problems such as IP address conflicts and stale gateway entries.
Purpose
arp exposes the ARP cache, the mapping every host keeps between the IP addresses of its neighbours and their MAC addresses. It is useful for confirming that the gateway resolves to the expected hardware, spotting IP conflicts, and listing devices the machine has recently talked to on the local segment.
Syntax
arp -a [inet_addr] [-N if_addr]
arp -d inet_addr [if_addr]
arp -s inet_addr eth_addr [if_addr]| Option | Effect |
|---|---|
-a | Display the cache (optionally for one address) |
-d | Delete an entry, or all with * |
-s | Add a static entry (rarely needed) |
-N if_addr | Show entries for a specific interface |
Viewing the cache
arp -aInterface: 192.168.1.37 --- 0xb
Internet Address Physical Address Type
192.168.1.1 a4-5e-60-1b-2c-3d dynamic
192.168.1.50 00-1c-42-2b-60-5a dynamic
192.168.1.255 ff-ff-ff-ff-ff-ff static
224.0.0.22 01-00-5e-00-00-16 staticThe first three bytes of a physical address identify the manufacturer, which helps recognise an unknown device.
Clearing the cache
arp -d *Run from an elevated prompt. Use it when a host's MAC has changed but the old mapping is still cached, which makes traffic to that host fail until the entry expires.
Diagnostic uses
- IP conflict: two devices claiming one address show alternating MACs; Windows also warns explicitly.
arp -afor the address reveals the current holder. - Gateway problems: if the gateway has no ARP entry after a
ping, the local link to the router is failing (cable, Wi-Fi, VLAN). - Device discovery: after pinging a subnet's broadcast or sweeping it,
arp -alists the hosts that answered, which works even when they ignore ICMP.
Common mistakes
- Adding static entries to "fix" problems. Static ARP is a niche tool; a stale dynamic entry is better cleared than pinned.
- Expecting remote hosts. ARP is local-link only; addresses beyond the router never appear.
PowerShell equivalent
Get-NetNeighbor shows the ARP and IPv6 neighbour caches as objects; Remove-NetNeighbor deletes entries.
Frequently asked questions
How do I clear the ARP cache?
From an elevated prompt, arp -d * clears all dynamic entries. The next packet to each host triggers a fresh ARP request. Use it when a device's MAC changed (for example a replaced router keeping the same IP) and traffic is going nowhere.
What does the "Type" column mean?
dynamic entries were learned automatically and expire; static entries were added by hand or by the system and persist. Broadcast and multicast addresses appear as static.