What is traceroute?

A command that finds the routers along the path to a destination, one by one. It sends packets while increasing the TTL in the IP header, 1, 2, 3 and so on, and from the ICMP Time Exceeded messages returned by the routers where the TTL reaches 0, it shows each router's address and round-trip time. On Windows the command is called tracert. Routers that do not return ICMP are shown as "*", which by itself does not mean there is a communication failure.

Japanese name: traceroute

traceroute appears in 3 practice questions across 2 topics, including Network Design and Operations, Computer Networking Basics.

Field: Network Design and Operations

Try it: questions about traceroute

In traceroute, what value is the TTL in the IP header set to in the packet sent to make the fourth router from the source return an ICMP Time Exceeded message?

Show answer and explanation

Answer: B. 4

Each router decreases the TTL by 1 when it forwards a packet, and when the TTL reaches 0 it discards the packet and returns an ICMP Time Exceeded message to the source. A packet sent with a TTL of 4 has 3 left after the first router, 2 after the second and 1 after the third, and reaches 0 at the fourth, so the fourth router returns the message. traceroute sends packets while raising the TTL one step at a time, 1, 2, 3 and so on, and from the source address and timing of the returned messages it shows the routers along the path and their delays in order. When packets reach the destination, it learns that it has finished from the destination's reply (an ICMP echo reply, or a message saying the destination port could not be reached).

Running traceroute from a host to web server 198.51.100.80 showed, on lines 1 to 5, 192.168.1.1, 10.0.0.1, "* * *" (no response), 203.0.113.1 and 198.51.100.80, in that order, and the web server could be used without problems. Which is the most appropriate interpretation of line 3?

AThe third router forwards packets but is configured not to return ICMP messages when the TTL reaches 0, or limits how many it returns
BPackets are discarded at the third router, and in fact none reach the fourth router or beyond
CPackets are encrypted only on the section through the third router, so its address cannot be read and * is shown

Show answer and explanation

Answer: A. The third router forwards packets but is configured not to return ICMP messages when the TTL reaches 0, or limits how many it returns

Responses come back from the routers from the fourth onward and from the destination, so packets are being forwarded past the third router. The third line shows * because that router is configured not to return ICMP Time Exceeded messages, or because it limits how often it sends ICMP to protect its own load. Each line of traceroute shows only the messages returned by the router at that position, so a * along the way does not necessarily mean there is a problem with communication. Conversely, if every line from some point on shows *, packets may be failing to get through around there.

traceroute in the explanations

traceroute sends packets while raising the TTL one step at a time, 1, 2, 3 and so on, and from the source address and timing of the returned messages it shows the routers along the path and their delays in order.
In traceroute, what value is the TTL in the IP header set t…
Each line of traceroute shows only the messages returned by the router at that position, so a * along the way does not necessarily mean there is a problem with communication.
Running traceroute from a host to web server 198.51.100.80 …
traceroute sends packets with TTL 1, 2, 3 and so on, and discovers the path from the time exceeded messages each router returns.
What is the role of the TTL (Time To Live) field in the IPv…

Questions that mention traceroute (3)

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