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ping requests not shown (if not sent) to non existent devices

asked 2020-01-27 00:13:37 +0000

DG12 gravatar image

updated 2020-01-28 21:42:28 +0000

If from I ping (the router) Wireshark shows both the request and the reply. If I ping (unassigned address) I expected to see the request but none is shown.

macOS Catalina 10.15.3 Please explain. Thank you

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answered 2020-01-27 00:50:23 +0000

Chuckc gravatar image

Assuming that .2 is on the same subnet as .1 and .23, there should have been an ARP request for If the device doesn't exist then there will not be an ARP response. If there is no entry for .2 in the ARP table then the ICMP request (ping) is not possible.

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Thanks. In other words: Devices that do not exist in a LAN, are not in ARP tables so pinging an IP address does not transmit.

ping (on macOS) stupidly (?) says "Request timeout for icmp_seq ."

on linux "Destination Host Unreachable" is slightly better

I changed the subject, hope that's OK

DG12 gravatar imageDG12 ( 2020-01-28 21:41:24 +0000 )edit

An alternative case is if the device you are pinging is NOT on the same subnet, i.e. needs to be routed, and is not present. In this case, you will see the ping request go out with no response if routing is setup correctly, which matches your original expectation. The way I look at: the stack needs a MAC address to build the frame; if the destination host is local, then arp will not complete so no MAC address is available, so the stack cannot put the frame on the wire (this is your sample case here, as already described in the answer). Conversely, if we need to route, the stack does have the destination MAC address: not of the target host, but of the next hop gateway (router). So the stack has what it needs to put the frame on the wire, and you would then see it.

Bob Jones gravatar imageBob Jones ( 2020-01-29 12:07:18 +0000 )edit

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Asked: 2020-01-27 00:13:37 +0000

Seen: 508 times

Last updated: Jan 28 '20