FNR is an experimental small domain block cipher for encrypting objects like IPv4 addresses, MAC addresses, arbitrary strings, etc. while preserving their input lengths. This is still an experimental block cipher, not ready for production yet. The FNR encryption scheme is a practical variant of Naor and Reingolds[1] work. The source code can be found on Github. It is a better fit in some scenarios than cryptdb, as the cryptdb method expands the data size.”]
Source: https://blogs.cisco.com/security/open-sourcing-fnr-an-experimental-block-cipher