Network Design and Operations
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.
- A. Correct. The routers beyond it and the destination respond, so the third router is forwarding and simply not returning messages.
- B. The fourth router and the destination respond, so packets do get past the third router.
- C. A * in traceroute means that no response came back within a set time; it has nothing to do with encryption.