The Address Resolution Protocol (ARP) enables a computer to convert IP addresses to the hardware addresses that the data-link layer protocol need to transmit frames. IP uses ARP to discover the hardware addresses to which each of its datagrams will be transmitted. To minimize the amount of network traffic ARP generates, the computer stores the resolved hardware addresses in a cache in system memory. The information remains in the cache for a short period (usually 2 to 10 minutes), in case the computer has additional packets to send to the same address.
Windows systems include a command-line utility called Arp.exe that you can use to manipulate the contents of the ARP cache.
For example, you can use Arp.exe to add to the cache the hardware addresses of computers you contact frequently, thus saving a little time and network traffic during the connection process. Addresses that you add to the cache manually are static, meaning that they are not deleted after the usual expiration period. The cache is stored in memory only, however, so it is erased when you reboot the computer. If you want to pre-load the cache whenever you boot your system, you can create a batch file containing Arp.exe commands and execute it from the Windows Startup group.
Arp.exe uses the following syntax:
ARP [-a {ipaddress}] [-n ipaddress] [-s ipaddress hwaddress {interface} [-d ipaddress {interface}]
- -a {ipaddress} This parameter displays the contents of the ARP cache. The optional ipaddress variable specifies the address of a particular cache entry to displayed.
- -n ipaddress This parameter displays the contents of the ARP cache, where ipaddress identifies the network interface whose cache you want to display.
- -s ipaddress hwaddress {interface} This parameter adds a new entry to the ARP cache, where the ipaddress variable contains the IP address of the computer, the hwaddress variable contains the hardware address of the same computer, and the interface variable contains the IP address of the network interface in the local system whose cache you want yo modify.
- -d ipaddress {interface} This parameter deletes the entry in the ARP cache that is associated with the computer represented by the ipaddress variable. The optional interface variable specifies the cache from which the entry should be deleted.
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