Results due to ICMP Filters
When performing a ICMP based traceroute, there are a few reasons for not receiving a reply. Packet loss on congested networks can explain away singular dropped replies. If all three probes for one or more links go missing, it's likely that a firewall is blocking ICMP or the routers are configured to not return an ICMP error. If however the traceroute ends with a successful reply after reporting multiple failed probes, then this is the result of a firewall configured to pass ping requests, and not traceroute.