|Name||CIDR block||Address range||Number of addresses||Classful description|
|24-bit block||10.0.0.0/8||10.0.0.0 - 10.255.255.255||Single Class A.|
|20-bit block||172.16.0.0/12||172.16.0.0 - 172.31.255.255||Contiguous range of 16 Class B blocks.|
|16-bit block||192.168.0.0/16||192.168.0.0 - 192.168.255.255||Contiguous range of 256 Class C blocks.|
Even though traffic to or from these addresses should never appear on the public Internet, it is not uncommon for such traffic to appear anyway. Some servers are configured (usually for logging reasons) to perform a reverse DNS lookup
These servers are registered in the DNS directory as the authoritative servers for the reverse lookup zone of the 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 addresses. These servers are configured to answer any query with a "nonexistent address" answer. This helps to reduce wait times because the (negative) answer is given immediately and thus no wait for a timeout is necessary. Additionally, the answer returned is also allowed to be cached by recursive DNS servers. This is especially helpful because a second lookup for the same address performed by the same node would probably be answered from the local cache instead of querying the authoritative servers again. This helps reduce the network load significantly. According to IANA, the blackhole servers receive thousands of queries every second. Because the load on the IANA blackhole servers became very high, an alternative service, AS112, has been created, mostly run by volunteer operators.
The AS112 project is a group of volunteer name server operators joined in an autonomous system. They run anycasted instances of the name servers that answer reverse DNS lookups for private network and link-local addresses sent to the public Internet. These queries are ambiguous by their nature, and can not be answered correctly. Providing negative answers reduces the load on the public DNS infrastructure.
Before 2001, the in-addr.arpa zones for the private networks were delegated to a single instance of name servers, blackhole-1.iana.org and blackhole-2.iana.org, called the blackhole servers. The IANA-run servers were under increasing load from improperly-configured NAT networks, leaking out reverse DNS queries, also causing unnecessary load on the root servers. The decision was made by a small subset of root server operators to run the reverse delegations; each announcing the network using the autonomous system number of 112. Later, the group of volunteers has grown to include many other organizations.
The name servers participating in the AS112 project are each configured to answer authoritatively for the following zones: