The JANET NRS (Name Registration Scheme) was a pseudo-hierarchical naming scheme adopted for use on British academic and research networks before the superficially similar system used by the Internet DNS had been fully established.
A principal difference was that the order of significance began with the most significant part (so called Big-endian addresses). Also, NRS names were canonically written in upper case. For example, the University of Cambridge had the NRS name UK.AC.CAM, whereas its DNS domain is cam.ac.uk. All NRS names had both a standard (long) and abbreviated (up to 18 characters) form. For example, UK.AC.CAMBRIDGE was the less widely used standard equivalent of the abbreviated name UK.AC.CAM.
Another significant difference from the DNS was the concept of context to name lookups, e.g. 'mail' or 'file transfer'. This made the NRS more sophisticated than the DNS, permitting overloading of names.
The NRS "second-level domains" consisted of UK.AC (JANET academic and scientific sites), UK.CO (commercial) and UK.MOD (Ministry of Defence). Any organisations not falling into these categories were given their own "second-level" name, e.g. UK.BL (British Library) or UK.NEL (National Engineering Laboratory).
For email, interoperability between the "Grey Book" email addressing style of user@UK.AC.SITE and Internet addresses of the style email@example.com was achieved by way of mail gateways. However, problems were caused when the least significant part of an Internet address matched the most significant part of an NRS address and vice-versa. (The classic joke was that e-mail intended for UK universities ended up in Czechoslovakia, since many JANET e-mail addresses were of the form user@UK.AC.universityname.CS, where "CS" stood for Computer Science (department), but ".cs" was also the two-letter country code for Czechoslovakia until 1995.)
JANET transitioned to using Internet protocols in the early 1990s, and the final mail gateway had been taken out of service by the end of 1997. Its one remaining legacy is the convention of using .uk for British DNS domains, rather than .gb, as specified by ISO 3166.