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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP Administration and Troubleshooting Utilities and Protocols

TCP/IP Route Tracing Utility (traceroute/tracert/traceroute6) 1 2 TCP/IP DNS Name Resolution and Information Lookup Utilities (nslookup, host and dig)

arp Utility Functions

To allow administrators to manage this ARP cache table, TCP/IP devices include an arp utility. It has three basic functions, which are invoked using three different versions of the command (which, for once, are the same in UNIX and Windows!):

• ARP Cache Table Display (“arp -a”): When the “-a” option is used with the utility, it displays the current contents of the ARP cache table. Each entry in the table shows the IP address and hardware address pair for one device (interface, actually); usually an indication is also given as to whether each entry is static or dynamic
  
  The exact format of the display varies from one implementation to the next; some programs show IP addresses while others show host names, and still others may show both. Some systems default to displaying host names but allow the “-n” option to also be used to force only IP addresses to be displayed and not names.
  
  
• ARP Cache Table Entry Addition (“arp -s <host-name> <hw-addr>”): This syntax allows an administrator to make a new manual ARP cache table entry that maps the given host name to the specified hardware address.
  
  
• ARP Cache Table Entry Deletion (“arp -d <host-name>”): This command removes the specified cache entry from the table. Some implementations allow the addition of another parameter to specify that all entries should be removed from the cache.

Certain versions of the software may also supplement these basic commands with additional features. One common additional option on UNIX systems is the ability to specify a file from which cache table entries may be read, using “arp -f <file-name>”. This saves a considerable amount of time and effort compared to typing each entry manually using “arp -s”.

Note also that access to options that cause the ARP cache table to be changed may be restricted by the operating system to only authorized users. This is especially true of the delete function, and especially especially true of the function that allows the entire ARP table to be deleted. J

TCP/IP Route Tracing Utility (traceroute/tracert/traceroute6) 1 2 TCP/IP DNS Name Resolution and Information Lookup Utilities (nslookup, host and dig)