Ping

Ping的返回訊息有： 　　Request Timed Out：這個訊息表示對方主機可以到達到TIME OUT，這種情況通常是為對方拒絕接收你發給它的數據包造成數據包丟失。大多數的原因可能是對方裝有防火牆或已下線。 　　Destination host Unreachable：這個訊息表示對方主機不存在或者沒有跟對方建立連線。這裡要說明一下"destination host unreachable"和"time out"的區別，如果所經過的路由器的路由表中具有到達目標的路由，而目標因為其它原因不可到達，這時候會出現"time out"，如果路由表中連到達目標的路由都沒有，那就會出現"destination host unreachable"。 　　Bad IP address：這個訊息表示你可能沒有連線到DNS伺服器所以無法解析這個IP位址，也可能是IP位址不存在。 　　Source quench received：訊息比較特殊，它出現的機率很少。它表示對方或中途的伺服器繁忙無法回應。 所以： Request time out：當所經過的路由器的路由表中具有到達目標的路由(理論上可達)，但是卻一直沒收到設備的回應，這時候會出現"Request time out"。也就是你ping傳送的封包理論上是能送達對方，卻沒收到回應的封包，有可能的障礙是：防火牆、設定不回應、回應的封包丟失(相關網路設定錯誤)等。(路是通的，只是沒回應) Destination host unreachable：當路由表中連到達目標的路由都沒有時，ping這個設備就會出現"Destination host unreachable"。也就是：router上沒有到達對方的路徑資訊，不 知道在哪裡，封包該往哪裡送。(找不到抵達對方的路) 不管是Destination host unreachable 和 Request time out可能原因都有很多，還有待整理和釐清。 Destination Host Unreachable This message indicates one of two problems: either 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 simply "Destination Host Unreachable," then there is no route from the local system, and the packets to be sent were never put on the wire. 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. 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.