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Table Of Contents  CertiGuide to Network+
 9  Chapter 0101:  TCP/IP
      9  XI  TCP/IP Network Diagnostic Utilities

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IPCONFIG
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ARP (Address Resolution Protocol)

This protocol maintains the computer’s list of MAC addresses and the corresponding IP address for each MAC address. The ARP program communicates over the network via the Arp protocol, to maintain and view the computer’s list that matches up MAC addresses and IP address. Remember that the MAC address is a unique address of a NIC.

To use ARP, type ARP at the Command Prompt. Using the appropriate switches (arguments or commands) will give you different information or perform different functions. To do this: type ARP /? And view the switch list. An example of the arguments (switches) is included in the following:

ARP

ARP –s
adds static address that remain until the machine is rebooted

ARP –a (or –g) displays the IP and MAC address, and inform you as to whether the mapping is static or dynamic

ARP –d manually deletes an entry

NT drops an ARP resolution that has not been used, after 2 minutes.


Figure 43: ARP /?

 


Normally, you would not need to use the arp command, because problems requiring that you inspect or modify the arp table are few and far between. However, here’s one situation in which arp would be useful.

You can use Arp to check for and resolve problems that might occur when multiple hosts end up with the same IP address. If the MAC address displayed for a particular IP address in your PC’s arp table (via arp –a) isn’t what you expect, it might be that you have two hosts on your network that have been assigned the same IP address. Of course, since the IP address is supposed to be a unique identifier of each host on the TCP/IP network, having multiple hosts with the same address is not a good thing. The symptom to a user of this is that a user tries to get to a machine by IP address and either cannot, or gets to the wrong machine (the one with the bad IP address). You can go to the machine that you think is at that IP address and type arp –N if_addr (where if_addr is the network interface number, normally 0) to display its MAC address. If it doesn’t match the one displayed by the arp –a command on your PC, you’ve found the problem.

How could you get two hosts with the same IP address? If you’re hard coding the IP address and other configuration information on each client, rather than handing out this information from a server via DHCP, it’s very possible that a typo was simply made when setting one or more PC’s information (or two techs working on updating IP addresses on a number of clients didn’t communicate with each other often enough, and re-used the same IP address!).


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XI  TCP/IP Network Diagnostic Utilities
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IPCONFIG
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