Reserved private IPv4 network ranges 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 77721616 Single Class A. 20-bit block 172.16.0.0/12 172.16.0.0 – 172.31.255.255 0485761 Contiguous range of 16 Class B blocks. 16-bit block 192.168.0.0/16 192.168.0.0 – 192.168.255.255 53665 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
- blackhole-1.iana.org (18.104.22.168)
- blackhole-2.iana.org (22.214.171.124)
- prisoner.iana.org (126.96.36.199)
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:
- For the 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 private networks:
- For the 169.254.0.0/16 link-local addresses:
- For unique identification purposes:
- Y. Rekhter; B. Moskowitz; D. Karrenberg; G. J. de Groot; E. Lear (February 1996). Address Allocation for Private Internets. Network Working Group. doi:10.17487/RFC1918. BCP 5. RFC 1918. https://tools.ietf.org/html/rfc1918. Updated by RFC 6761.
- J. Abley; W. Maton (July 2011). I'm Being Attacked by PRISONER.IANA.ORG!. IETF. doi:10.17487/RFC6305. ISSN 2070-1721. RFC 6305. https://tools.ietf.org/html/rfc6305.
- T. Hardie (April 2002). Distributing Authoritative Name Servers via Shared Unicast Addresses. Network Working Group IETF. doi:10.17487/RFC3258. RFC 3258. https://tools.ietf.org/html/rfc3258.
- J. Abley; W. Sotomayor (May 2015). AS112 Nameserver Operations. IETF. doi:10.17487/RFC7534. RFC 7534. https://tools.ietf.org/html/rfc7534. Obsoletes RFC 6304.
- J. Abley; B. Dickson; W. Kumari; G. Michaelson (May 2015). AS112 Redirection Using DNAME. IETF. doi:10.17487/RFC7535. RFC 7535. https://tools.ietf.org/html/rfc7535.
- S. Cheshire; B. Aboba; E. Guttman (May 2005). Dynamic Configuration of IPv4 Link-Local Addresses. Network Working Group IETF. doi:10.17487/RFC3927. RFC 3927. https://tools.ietf.org/html/rfc3927.