Worth noting that traceroute isn't always useful. traceroute uses ICMP - the protocol that can't deside which layer to live on (ok - I know it was a lame joke, but I couldn't resist).

Many people risk breaking discovery rather than allowing ICMP to traverse unchecked. Ever not been able to ping a host that you knew was there...? ICMP message types are explained in RFC 792. http://www.faqs.org/rfcs/rfc792.html

Cheers,

Keith