The tracert command (called traceroute in Unix/Linux) is an important diagnostic tool for TCP/IP networks. It allows you to determine the list of all routers through which a data packet passes from your computer to another device on the network or the Internet. Each such router is called a “hop,” probably because the creators of the IP protocol jokingly compared it to a game of hopscotch. If there is a problem connecting to another computer, tracert helps to pinpoint exactly where on the route the problem occurs.
The tracert command makes three attempts to send a packet to each router and displays the response time for each attempt. It then displays the DNS name of the router (if available) and its IP address. To use tracert, you need to enter a command specifying the host name or IP address of the computer to which you want to trace the route. For example, if you are having problems sending mail to the abc.com server, after determining its mail server using nslookup, you can use tracert to view the route to that server.
During tracert, packets may encounter timeouts, which are displayed as asterisks instead of response times. Sometimes this is due to temporary glitches, so it makes sense to repeat the check. If the timeout repeats on the same router, it may indicate a real problem on that section of the network.
The principle of tracert is based on the TTL (Time to Live) field in an IP packet. TTL is usually understood as “lifetime,” but in the context of IP, it indicates how many routers a packet can pass through before reaching its destination. Each router decreases the TTL by one. When the TTL value reaches zero, the router refuses to forward the packet and sends back a “Time Exceeded” message. The tracert command uses a series of special ICMP Echo Request messages with gradually increasing TTL. First, the TTL is set to 1, the packet reaches the first router, the TTL decreases to zero, and the router sends a time exceeded message. tracert records the router’s IP address and response time. Next, the TTL is increased to 2, the packet passes through the first router and reaches the second, and the process is repeated until the packet reaches its final destination.
This process allows you to clearly see which routers the data passes through and how long each “hop” takes, which helps identify problem areas in the network. In Unix/Linux, the traceroute command performs similar actions using a slightly different set of TCP/IP messages, but with the same goal—to track the route of packets to their destination.
