Troubleshooting hardware addresses by using arp

 

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The Address Resolution Protocol (ARP) allows a host to find the media access control address of a host on the same physical network, given the IP address of the host. To make ARP efficient, each computer caches IP-to-media access control address mappings to eliminate repetitive ARP broadcast requests.

You can use the arp command to view and modify the ARP table entries on the local computer. The arp command is useful for viewing the ARP cache and resolving address resolution problems.

 

To view the Address Resolution Protocol (ARP) cache

1. Open Command Prompt
2. At the command prompt, type arp -a.

   For example, if you recently used the ping command to test and verify connectivity from this computer to a host computer at IP address 10.0.0.99, the ARP cache displays the following entry:

   
   Interface: 10.0.0.1 on Interface 0x1
     Internet Address      Physical Address      Type
     10.0.0.99             00-e0-98-00-7c-dc     dynamic
   

   In this example, the cache entry indicates that the remote host computer at 10.0.0.99 resolved to a media access control address of 00-e0-98-00-7c-dc assigned in the network adapter hardware of the remote computer. The media access control address is the address that your computer used to physically communicate with this remote TCP/IP host on your network.

 Note

• To open Command Prompt, click Start, point to Programs, point to Accessories, and then click Command Prompt.
• To minimize ARP broadcast traffic on your network, Windows 2000 maintains a cache of hardware-to-software address mappings for future use. This cache contains the following two types of entries:
  • Dynamic ARP cache entries

    These entries are added and deleted automatically during normal use of TCP/IP sessions with remote computers. Dynamic entries age and expire from the cache if not reused within 2 minutes. If a dynamic entry is reused within 2 minutes, it may remain in the cache and age up to a maximum cache life of 10 minutes before being removed or requiring cache renewal by using the ARP broadcast process.

  • Static ARP cache entries

    These entries are added manually by using the arp command with the -s option. Static entries remain in the ARP cache until the computer is restarted.

To add a static ARP cache entry

1. Open Command Prompt
2. At the command prompt, type:

   arp -s ip_address mac_address

   where:

   ip_address	Specifies the IP address of a local (on the same subnet) TCP/IP node.
   mac_address	Specifies the media access control address for a network adapter installed and used on the local TCP/IP node.

   For example, to add a static ARP entry for a local TCP/IP node with an IP address of 10.0.0.200 that resolves to a media access control address of 00-10-54-CA-E1-40, you type the following at a command prompt:

   arp -s 10.0.0.200 00-10-54-CA-E1-40

 Note

• To open Command Prompt, click Start, point to Programs, point to Accessories, and then click Command Prompt.
• Static ARP entries can help speed access for hosts that are frequently used.
• Static entries are valid only until Windows 2000 is restarted. To make static ARP cache entries persistent, you can add arp commands to a batch file that is run at system startup.