Linux network troubleshooting and debugging

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http://unix.stackexchange.com – From time to time Linux and Unix users faced with various network problems. Many of these problems are presented here and at some others troubleshooting forums, but the are very concrete and contains a lot of additional technical information, and sometimes it's rather difficult to understand the main point and the real reason of buggy system behavior. By asking this question, my intention is to start a community wiki page which allows to generalize our network troubleshooting and debugging experience. I hope the Linux and Unix users could easier recognize and solve("divide and conquer") thei (HowTos)