arp

arp

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Resolves IP addresses into media access control (MAC) addresses and caches them for reuse.

Syntax

 arp -s   IPaddress MACaddress   [   interfacenumber   ] arp -d   IPaddress   [   interfacenumber   ] arp -a [   IPaddress   ] [-N   interfacenumber   ] 

Options

-a [IPaddress]

Resolves the specified IP address into its associated MAC address by querying the Address Resolution Protocol (ARP) cache on the local machine. (If no address is specified, all cached IP-to-MAC address mappings are displayed.)

-g [IPaddress]

Same as -a .

-N interfacenumber

Specifies the network adapter whose ARP cache is to be queried. (Each network adapter has its own ARP cache on a multihomed machine.) Use arp -a to determine the number of each interface. If arp is used without -N on a multihomed machine, the first interface found is used.

-d IPaddress [interfacenumber]

Removes the IP-to-MAC address mapping from the local ARP cache for the specified IP address and interface. If no IPaddress is specified, the top entry in the ARP cache is removed.

-s IPaddress MACaddress [interfacenumber]

Adds a static IP-to-MAC address mapping to the local ARP cache for the specified interface. The MAC address must be expressed in hexidecimal form as 12 characters , in groups of 2, separated by dashes. Static ARP mappings are persistent until the system reboots.

Examples

View the ARP cache on the local machine:

  arp -a  Interface: 172.16.11.104 on Interface 0x2   Internet Address      Physical Address      Type   172.16.11.100         00-40-95-d1-29-6c     dynamic 

Ping the host named Leonardo to determine its IP address, and add a mapping for it to the local ARP cache:

  ping -n 1 leonardo  Pinging leonardo [172.16.11.39] with 32 bytes of data: Reply from 172.16.11.39: bytes=32 time<10ms TTL=32 Ping statistics for 172.16.11.39:   Packets: Sent = 1, Received = 1, Lost = 0 (0% loss), Approximate round trip times in milli-seconds:   Minimum = 0ms, Maximum =  0ms, Average =  0ms 

Verify that an IP-to-MAC address mapping for Leonardo (172.16.11.39) has been added to the local ARP cache:

  arp -a  Interface: 172.16.11.104 on Interface 0x2   Internet Address      Physical Address      Type   172.16.11.39          00-40-95-d1-32-90     dynamic   172.16.11.100         00-40-95-d1-29-6c     dynamic 

Add a static mapping for Leonardo to the local ARP cache:

  arp -s 172.16.11.39 00-40-95-d1-32-90  

Verify the static mapping:

  arp -a  Interface: 172.16.11.104 on Interface 0x2   Internet Address      Physical Address      Type   172.16.11.39          00-40-95-d1-32-90     static   172.16.11.100         00-40-95-d1-29-6c     dynamic 

Notes

• For one host to communicate with another on a TCP/IP network, the first host uses arp to resolve the second host's IP address into its corresponding MAC address. This MAC address then provides a destination address for Ethernet or token ring frames sent from the first host to the second. arp caches these IP-to-MAC address mappings for a short time (from 2 to 10 minutes) to reduce the number of ARP broadcasts needed.

• arp is a useful tool for troubleshooting TCP/IP networks because it can be used to find the MAC address of any host on the local subnet, provided that the IP address for the host is known.

• arp can be used to view the ARP cache only on the local machine, not on remote ones.

• To reduce broadcast traffic and speed up TCP/IP communications, you can add static mappings to the ARP cache on client machines. This lets clients resolve IP addresses of commonly used servers on the network from the clients ' local ARP caches instead of using ARP broadcasts.

• To make static ARP cache mappings persistent across reboots, add arp commands to a batch file and run the file at system startup.

• Gratuitous ARP (or " courtesy ARP") is a TCP/IP mechanism used by hosts to announce their IP address, which avoids duplicate IP addresses on the network. You can disable gratuitous ARP by editing the registry and specifying 1 for the following value:

  HKLM\System\CurrentControlSet\Services\TcpIp\Parameters\ArpRetryCount

See Also

ipconfig , pathping , ping , TCP/IP , tracert