In the mid-1970s, the British Post Office Telecommunications division (BPO-T) worked with the academic community in the United Kingdom and the computer industry to develop a set of standards to enable interoperability among different computer systems based on the X.25 protocol suite for packet-switched wide area network (WAN) communication.
First defined in 1975,[1] the standards evolved through experience developing protocols for the NPL network in the late 1960s and the Experimental Packet Switched Service in the early 1970s.
[12] From late 1991, Internet protocols were adopted on the Janet network instead; they were operated simultaneously for a while, until X.25 support was phased out entirely in August 1997.
It really occupies the top of the Network layer, making up for X.25's lack of NSAP addressing at the time, which did not appear until the X.25 (1980) revision, and was not available in implementations for some years afterward.
Over time, as technology evolved, many of the concepts and principles from the Coloured Book Protocols were integrated into broader international standards.
One famous quirk of Coloured Book was that components of hostnames used reverse domain name notation as compared to the Internet standard.