Even though TCP/IP provides a means of communicating the delivery failure back to the sender via ICMP, traffic destined for such addresses is often just dropped.
Most firewalls (and routers for household use) can be configured to silently discard packets addressed to forbidden hosts or ports, resulting in small or large "black holes" in the network.
Personal firewalls that do not respond to ICMP echo requests ("ping") have been designated by some vendors[3] as being in "stealth mode".
Remote Triggered Black Hole Filtering (RTBH) is a technique that provides the ability to drop undesirable traffic before it enters a protected network.
Null routing has an advantage over classic firewalls since it is available on every potential network router (including all modern operating systems), and adds virtually no performance impact.
Many email systems operators and users[13] consider DNSBLs a valuable tool to share information about sources of spam, but others including some prominent Internet activists have objected to them as a form of censorship.
[14][15][16][17] In addition, a small number of DNSBL operators have been the target of lawsuits filed by spammers seeking to have the lists shut down altogether.
[18] Some firewalls incorrectly discard all ICMP packets, including the ones needed for Path MTU discovery to work correctly.