Networking debuging

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http://serverfault.com – How to systematically find culprit behind loss of internet in linux? I do know that to to download files/access webpage/use ssh/etc , there are several layers of communication made: application,transport,internet,link ( wikipedia ) I don't know How each of these layers (excluding application layer) map to programs/components in linux? How to check each of these components efficiently - if they are working correctly? In which order to check them My goal is to have (reasonably) simple system for tracking down the problem. In pseudo code it would look something like for(component in A. (HowTos)