Suppose, My host is A

A(my machine) has ip address 12.

B has ip address 13

C has ip address 14

My machine has below entries in ARP cache

B => ip address 13 and its corresponding MAC address

C => ip address 14 and its corresponding MAC address

Now, B and C disconnects from LAN and when they are connected back to LAN, but this time C is assigned IP address 13 and B is assigned IP address 14

But my ARP cache has old info. What would happen if I try to ping B now ?

2 Answers 2


If that were to happen just as you state, then your pings would fail.

In practise if you are using DHCP to hand out addresses, the server will give you back the same address. Even if it doesn't (because you clear your DHCP server cache and reboot it, all before your ARP cache times out) you'll probably find that your B and C will send gratuitous ARPs anyway to update the ARP cache.

  • gotta explore what are gratuitous ARPs..thanx for the info
    – Abhinav
    Apr 26, 2017 at 8:56
  • @Abhinav This might help: pracnet.net/arp
    – Eddie
    Apr 27, 2017 at 17:11

The ICMP echo request generated by our PC will get to C but it will discard it because its destination IP address won't match with one assigned to its network card, it will say "it got to me but it's not for me, I won't process it".

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