Ping does not pass a Web site but can access the site

Visit a website, want to see its IP is how much, on the Windows command line went down to ping the next site, the IP address is resolved, but found that the request timed out, there is no ping, then very confused. And then tracert the IP tracking to see what the problem, and finally found that when the IP was traced to the request was rejected. All of a sudden, it should be the server set up the relevant policies to the network layer ICMP echo Request messages are limited, and access to the Web page with the HTTP protocol, so this behavior occurs.



The nature of the ping: sends an ICMP echo request message to the destination host, and waits for the Echo echo of the ICMP reply. The echo message is then printed out. echo results include: Number of bytes, reaction time, TTL (Time to live)

The essence of Tracert: Using the TTL value in the header of the ICMP datagram and IP datagram. The TTL (Time to live) is the lifetime of an IP datagram, and when each IP datagram passes through the router, the TTL value is subtracted by 1 or the time spent in the router is subtracted, but most datagrams stay in the router for less than 1 seconds. So it's actually subtracting 1 from the TTL value. In this way, the TTL value is equivalent to the counter of a router.
When the router receives an IP datagram with a TTL of 0 or 1, the router no longer forwards the data, but discards it directly and sends an ICMP "timeout" message to the source host. The key to the TRACERT program is the IP address of the IP header of the echo-echo ICMP message that is the router. At the same time, if the destination host is reached, we do not know, so, tracert also send a UDP message to the destination host, and select a very large value as a UDP port, so that any application of the host does not use this port. Therefore, when the destination host is reached, the UDP module produces a "Port unreachable" error, so that you can determine whether the destination is reached.