tracert 命令详解

How to Use the TRACERT Utility

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.

TRACERT prints out an ordered list of the intermediate routers that return ICMP "Time Exceeded" messages. Using the -doption with the tracert command instructs TRACERT not to perform a DNS lookup on each IP address, so that TRACERT reports the IP address of the near-side interface of the routers.

In the following example of the tracert command and its output, the packet travels through two routers (157.54.48.1 and 11.1.0.67) to get to host 11.1.0.1. In this example, the default gateway is 157.54.48.1 and the IP address of the router on the 11.1.0.0 network is at 11.1.0.67.

The command:

C:\>tracert 11.1.0.1

The output from the command:

   Tracing route to 11.1.0.1 over a maximum of 30 hops

   ---------------------------------------------------

   1      2 ms       3 ms       2 ms      157.54.48.1

   2     75 ms      83 ms      88 ms      11.1.0.67

   3     73 ms      79 ms      93 ms      11.1.0.1

   Trace complete.

How to Use TRACERT to Troubleshoot

You can use TRACERT to find out where a packet stopped on the network. In the following example, the default gateway has found that there is no valid path for the host on 22.110.0.1. Probably, either the router has a configuration problem, or the 22.110.0.0 network does not exist, reflecting a bad IP address.

The command:

C:\>tracert 22.110.0.1

The output from the command:

   Tracing route to 22.110.0.1 over a maximum of 30 hops

   -----------------------------------------------------

   1  157.54.48.1  reports: Destination net unreachable.

   Trace complete.
TRACERT is useful for troubleshooting large networks where several paths can lead to the same point or where many intermediate components (routers or bridges) are involved.

How to Use TRACERT Options

There are several command-line options that you can use with TRACERT, although the options are not usually necessary for standard troubleshooting. 

The following example of command syntax shows all of the possible options:

tracert -d -h maximum_hops -j host-list -w timeout target_host

What the parameters do:

 -d

      Specifies to not resolve addresses to host names

   -h maximum_hops

      Specifies the maximum number of hops to search for the target

   -j host-list

      Specifies loose source route along the host-list

   -w timeout

      Waits the number of milliseconds specified by timeout for each

      reply

   target_host

      Specifies the name or IP address of the target host