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Traceroute (tracert) works by sending a packet to an open UDP port on a destination machine. For the initial three packets, traceroute sets the TTL (see explanation of TTL) to 1 and releases the packet. The packet then gets transferred to the first router (completing the first hop), and the TTL gets decremented by the router from 1 to 0. The router then discards the packet and sends off an ICMP notification packet to the original host with the message that the TTL expired from the router. This tells tracert what the first hop is and how long it takes to get there. Traceroute repeats this, gradually incrementing the TTL until a path to the remote host is traced and it gets back an ICMP Port Unreachable message, indicating that the remote host has been reached.
Response times may vary dramatically because the packet is crossing long distances, other times the increases come from network congestion.
Example of a TracerouteC:> tracert www.linux.org or C:> tracert 188.8.131.52
Tracing route to www.linux.org [184.108.40.206] over a maximum of 30 hops: 1 <10 ms <10 ms <10 ms mn-bldg-rtr-vlan200-3.gw.more.net [220.127.116.11] 2 <10 ms <10 ms <10 ms co-r12-01-atm0-0-10.mo.more.net [18.104.22.168] 3 <10 ms 10 ms <10 ms kc-r12-01-atm1-0-131.mo.more.net [22.214.171.124] 4 <10 ms 10 ms <10 ms bb2-g8-0.kscymo.swbell.net [126.96.36.199] 5 <10 ms 10 ms 10 ms sl-gw9-kc-2-0.sprintlink.net [188.8.131.52] 6 * * * 7 50 ms 61 ms 60 ms 198.ATM7-0.XR2.TOR2.ALTER.NET [184.108.40.206] 8 50 ms 60 ms 60 ms 194.ATM7-0.GW1.TOR2.ALTER.NET [220.127.116.11] 9 50 ms 70 ms 60 ms att2-gw.customer.alter.net [18.104.22.168] 10 61 ms 60 ms 60 ms pos5-0-0.hcap1-ott.bb.attcanada.ca [22.214.171.124] 11 60 ms 70 ms 70 ms 126.96.36.199 12 60 ms 81 ms 70 ms router.invlogic.com [188.8.131.52] 13 70 ms 70 ms 80 ms www.linux.org [184.108.40.206] Trace complete.
Note the asterisks on line six. This can indicate that a response wasn't received. Some routers do not issue TTL-expired ICMP messages.
How to perform a Traceroute
Traceroute can be accessed at a DOS or command prompt. An Internet connection must already be established.
- In a Windows 2000 or XP environment, click on Start > Run. Type cmd into the dialog box, then click OK.
- In the resulting command line window, type tracert hostname, where hostname can be a domain name, a machine name or an IP address.
- Press Enter.
Mac OS X Environment
- Double-click the Hard Drive icon > Applications folder > Utilities folder > Network Utility program.
- Select the Traceroute tab and enter the hostname, where hostname can be a domain name, a machine name or an IP address.
- Press Enter.
- Launch a command line interface (will vary depending on the operating system distribution).
- In the resulting command line window or screen, type traceroute hostname, where hostname can be a domain name, a machine name or an IP address.
- Press Enter.
Additional Traceroute CommandsThese extra commands will work in any environment except for the Novell operating system:
- To stop traceroute, type Ctrl-C.
- To print the results of traceroute to a text file on a local drive, add "> textname.txt" to the command.
tracert www.example.com > tracert.txt
will save the results to a tracert.txt file located on the primary hard drive.
- To append the results of more than one traceroute to the same text file on your local drive add >> tracert.txt to the command:
tracert www.example.com >> tracert.txt
will save the results to the same tracert.txt file located on the primary hard drive.