X.25

X.25 started as a research project in England’s National Physical Laboratory, governed by Donald Davies, who developed the concept of packet-switching networks. During the late 1960s an experimental network called SERCnet (Science Engineering Research Council Network) was established for this project. After the integration of many new features and extensions into the system a description in form of a series of technical books was released. Due to the different colours of the book covers, these descriptions became known as “colour books” outlining a concept of what would finally become the X.25 standard.
The main idea of X.25 was to establish a packet-switching network within a then unreliable telephone network.

A big part of the X.25 system was the description of strict error correction. X.25 had all properties of a circuit switched network although internally the files were transmitted via packet-switching. Connected computers had a telephone number-like address in the X.121 format. The so-called Network User Address (NUA) consisted of a three-digit country code, a one-digit provider code and a ten-digit national terminal number.
X.25 was used as a dedicated line for permanent connections between computers for quite a while. X.25 networks worldwide are still available, but their use decreases steadily.