When examining the topology of the network, the black holes themselves are invisible, and can only be detected by monitoring the lost traffic; hence the name.
The most common form of black hole is simply an IP address that specifies a host machine that is not running or an address to which no host has been assigned.
Even though TCP/IP provides means of communicating the delivery failure back to the sender via ICMP, traffic destined for such addresses is often just dropped.
Note that a dead address will be undetectable only to protocols that are both connectionless and unreliable (e.g., UDP). Connection-oriented or reliable protocols (TCP, RUDP) will either fail to connect to a dead address or will fail to receive expected acknowledgements.
Firewalls and “stealth” ports
Black hole filtering
Black hole filtering refers specifically to dropping packets at the routing level, usually using a routing protocol to implement the filtering on several routers at once, often dynamically to respond quickly to distributed denial-of-service attacks.
PMTUD black holes
Black hole e-mail addresses
A black hole e-mail address is an e-mail address which is valid (messages sent to it will not generate errors), but to which all messages sent are automatically deleted, and never stored or seen by humans. These addresses are often used as return addresses for automated e-mails.