Which of the following is **not** a reason why a ping might timeout? 1. A router

Question

Which of the following is **not** a reason why a ping might timeout?

1. A router in between is filtering all ICMP traffic.

2. The destination has a firewall that will not accept ICMP traffic.

3. The sending system will not respond to ICMP requests.

4. The destination system is powered off.

I know this question has been posted before by several people, but the lack of a good explaination and several different answers is why I am reposting this question. If you are going to answer this would you please explain why that is the correct answer. I am trying to learn this and for some reason cannot pick it up from the book, thank you!

Explanation / Answer

The most suited answer for this is option 3rd, i.e.,

The sending system will not respond to ICMP requests.

You see a ping is an ICMP packet. For comparison, web traffic or HTTP, is generally a TCP packet on port 80. A given server may be providing responses for the purpose of web traffic, but blocks ICMP packets or the destination router might be using a firewall for the same reason. These filtering and blocking are mostly used to avoid hacking using ICMP packets, which is a very common way to perform DoS attacks. This covers option 1 and option 2 as valid reasons for a ping timout.

Coming to the 4th option, obiously a system needs to be connected to a network to recieve the ping packets sent by a sending system. If the destination system is turned off, means it's not directly connected to the network and so is not able to receive an ICMP echo packet or to send back an ICMP echo reply packet.

This leaves us with option 3rd as the only invalid reason for a ping timeout.

Hope this helps..

BEST WISHES!

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