What is Arp-a

Posted on 2007-09-29
Medium Priority
Last Modified: 2008-01-09

When we type Arp-a we get some data.What does this mean.

Question by:bsharath
LVL 12

Expert Comment

ID: 19983366
 -a            Displays current ARP entries by interrogating the current
                protocol data.  If inet_addr is specified, the IP and Physical
                addresses for only the specified computer are displayed.  If
                more than one network interface uses ARP, entries for each ARP
                table are displayed.
  -g            Same as -a.
  inet_addr     Specifies an internet address.
  -N if_addr    Displays the ARP entries for the network interface specified
                by if_addr.
  -d            Deletes the host specified by inet_addr. inet_addr may be
                wildcarded with * to delete all hosts.
  -s            Adds the host and associates the Internet address inet_addr
                with the Physical address eth_addr.  The Physical address is
                given as 6 hexadecimal bytes separated by hyphens. The entry
                is permanent.
  eth_addr      Specifies a physical address.
  if_addr       If present, this specifies the Internet address of the
                interface whose address translation table should be modified.
                If not present, the first applicable interface will be used.
  > arp -s   00-aa-00-62-c6-09  .... Adds a static entry.
  > arp -a                                    .... Displays the arp table.
LVL 29

Accepted Solution

Alan Huseyin Kayahan earned 1000 total points
ID: 19983371
  Hi Sharath
     ARP is used to perform IP address resolution which is the linking of IP addresses to MAC (Media Access Control) hardware addresses. ARP uses a broadcast to do this by asking the host that has the given IP address to respond to the broadcast with its MAC address. This step is required to create the data packet at the data link layer of the OSI network model.
   ARP -a or -q lists the arp cache. The output you see is your gateway address and its MAC.

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Assisted Solution

ibu1 earned 500 total points
ID: 19983373
In short arp -a will give the mac address or the network card address of the interface.Example
C:\Documents and Settings\user.ORACLE7.000>arp -a

Interface: --- 0x2
  Internet Address      Physical Address      Type         00-16-76-7b-21-2e     dynamic

More precise,
Address Resolution Protocol
   <networking, protocol> (ARP) A method for finding a host's
   Ethernet address from its Internet address.  The sender
   broadcasts an ARP packet containing the Internet address
   of another host and waits for it (or some other host) to send
   back its Ethernet address.  Each host maintains a cache of
   address translations to reduce delay and loading.  ARP allows
   the Internet address to be independent of the Ethernet address
   but it only works if all hosts support it.

   ARP is defined in RFC 826.

   The alternative for hosts that do not do ARP is constant

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Expert Comment

by:Alan Huseyin Kayahan
ID: 19983402
   No it doesnt give the mac address of the interface. In above example, the MAC address does not belong to your interface. Type ipconfig /all and check. In above example, is your gateway, and shown MAC address belongs to it.
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Author Comment

ID: 19983406

In simple words.
Arp -a shows all connections that are made at the moment?
Am i right
LVL 29

Expert Comment

by:Alan Huseyin Kayahan
ID: 19983412
   Hi Sharath
        No. The command you lookfor is netstat -n

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Expert Comment

by:Alan Huseyin Kayahan
ID: 19983414
  If you want to see host names instead ips, type only netstat without -n
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Assisted Solution

KCTS earned 500 total points
ID: 19983422
in short
arp -a
lists the current IP to MAC cache on the local computer. This will be the IP address and corresponding MAC address of computers contacted recently.

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