Difference Between Host Unreachable And Request Timed Out

1 minute read

Description:

So I have seen this many times when troubleshooting network issues and decided to research it a little further:

 

To Resolve:

  1. “Destination host unreachable” is an ICMP message generated by a router when it believes it has a message for a local network but it can’t find a host to claim that message.

When the first ping hits the router nearest to the destination, that router begins sending ARP request packets on its local network, asking for the MAC address of a host with an IP address matching the ping’s destination. While the router is waiting for a response, the pings timeout. Only once the ARP times out does the router decide the host is not reachable. At that point it caches the result and begins sending back the ICMP Destination host unreachable messages to any new network traffic that comes along. Exactly how long it will cache the ARP failure depends on the router. The host is saying “I can’t route your packet, the destination address looks like it is on my subnet, but I can’t ARP it”.

Essentially, the local system has no route to the desired destination or a remote router reports that it has no route to the destination. If the message is “Reply From < IP address >: Destination Host Unreachable,” then the routing problem occurred at a remote router, whose address is indicated by the “< IP address >” field.

  1. “Request Timed Out” – This message indicates that no Echo Reply messages were received within the default time of 1 second. This can be due to many different causes; the most common include network congestion, failure of the ARP request, packet filtering, routing error, or a silent discard.

References:

http://technet.microsoft.com/en-us/library/cc940095.aspx