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I am trying to understand how tracert works,

  1. I did a ping www.google.co.nz, found out Google's server IP is 172.217.25.131
  2. tracer 172.217.25.131
  3. I got the following screen of information

enter image description here

Can I please ask why there are three Request timed out?

  • Are step 3,4,5 failed attempts on one router or on three different routers?

1 Answer 1

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Extract from Microsoft Support How to Use TRACERT to Troubleshoot TCP/IP Problems in Windows

The TRACERT diagnostic utility determines the route to a destination by sending Internet Control Message Protocol (ICMP) echo packets to the destination. In these packets, TRACERT uses varying IP Time-To-Live (TTL) values. Because each router along the path is required to decrement the packet's TTL by at least 1 before forwarding the packet, the TTL is effectively a hop counter. When the TTL on a packet reaches zero (0), the router sends an ICMP "Time Exceeded" message back to the source computer.

TRACERT sends the first echo packet with a TTL of 1 and increments the TTL by 1 on each subsequent transmission, until the destination responds or until the maximum TTL is reached. The ICMP "Time Exceeded" messages that intermediate routers send back show the route. Note however that some routers silently drop packets that have expired TTLs, and these packets are invisible to TRACERT.

So you can see that tracert rely on the router sending back an ICMP message "Time Exceeded".

Many routers on the Internet

  • either are configured to not send ICMP message, those routers won't answer to ping
  • either silently drop a TTL expired packet (but may respond to ping)

In both case, tracert doesn't receive any answer and display "Request time out".

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