On some networks they are filtered; so you cannot get a response

ICMP filtering is not as common-place as it was say 2-3 years ago. There was one particular worm (can't recall exactly which one.. MSBlaster, perhaps?) that used an ICMP probe to search for vulnerable hosts. It caused havoc for a short time, and many ISP's started filtering ICMP in response. The problem with ICMP filtering is that it can have unwanted side-effects, such as breaking Path MTU Discovery - and so many of these filters have gradually been removed.

In my experience, most ISP's will will remove an ICMP filter, or at least allow it for a specific host - if asked nicely.

Cheers,
Darren :)

In reply to Re^2: Pinging network devices and setting SNMP traps by McDarren
in thread Pinging network devices and setting SNMP traps by jimbus

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