ARP (address resolution protocol), the default protocol that performs the conversion from an IP address to an Ethernet address.   The ARP protocol is included with Linux systems, and is installed by default as part of the TCP/IP network software.   You do not need to do anything to activate it.

  The protocol dynamically builds a table that maps IP addresses to Ethernet addresses via broadcast facilities of the Ethernet.   Try to Ping an address to get a complete or info regarding so

-a view the entire table with MAC address, IP, network interface hostname show's the individual host entry -n will show two(2) extra fields (1) Mask and (2) Flags -s command-line argument tells ARP that this is a static entry pub this argument says that this entry will be published

Flags field can contain three(3) possible values

C this is a complete entry.   To be valid, an entry must be complete M indicates a static entry that was manually entered.   ARP entries are normally dynamic.

P indicates an entry that will be published.   In other words, IF this computer receives an ARP broadcast for the IP address in this entry, the local host responds with the Ethernet address, even though the IP address does not really belong to the local host.

ex.: 

  # arp -s killdeer 00:00:C0:4F:3E:DD pub

  # arp -s lark 00:00:C0:4F:3E:DD pub

Enabling IP Packet Forwarding

  In general, host do not forward datagrams, but routers must.   To use a Linux system as a router, enable IP forwarding by setting the correct value in the /proc/sys/net/ipv4/ip_forward file.   set this the value to one(1) to enable it.

  $ echo "1" > /proc/sys/net/ipv4/ip_forward

  $ cat /proc/sys/net/ipv4/ip_forward