Search the most specific recorded network first
07The historical IPv4 fallback discovers network/subnet names into a stack, then checks the most specific name first. This is not a modern CIDR or IPv6 geolocation algorithm.
Pop lab.example.org next; its LOC is present. Use it as an approximation and stop. Even if campus.example.org has LOC, it does not replace this more-specific subnet result.
Source for this drawing: RFC 1876 · Read RFC section 5.2.3 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
The specified algorithm starts from a class-A/B/C host-zero address, queries reversed-octet IN-ADDR.ARPA for PTR and A, pushes the returned name, and uses any returned A value as the next subnet mask.
- The specified algorithm starts from a class-A/B/C host-zero address, queries reversed-octet IN-ADDR.ARPA for PTR and A, pushes the returned name, and uses any returned A value as the next subnet mask.
- When no further mask A record is present, pop names from most specific to least specific until a LOC record is found. A subnet result takes priority over a broader network result.
- The fictional 172.20.2.17 fixture uses the historical class-B start and masks 255.255.255.0 then 255.255.255.240. A records at these special nodes are masks, not host address answers. This RFC defines no extensions for other addressing schemes.
Examples are fictional, offline teaching fixtures. RFC1876 is Experimental; its classful IPv4 fallback is historical, not universal current geolocation. Follow the linked official sections for full requirements.
Read the RFC section ↗