A protocol is a formal description of message formats and the rules two or more machines must follow to exchange those messages. Because TCP (transmission control protocol) and IP (Internet protocol) are the two most fundamental protocols, the entire protocol suite that is used by the Internet is often referred to as TCP/IP. X.25 is the CCITT (Consultative Committee on International Telephony and Telegraphy) standard protocol employed by Telnet, and is most popular in Europe. Ethernet utilizes CSMA/CD (carrier sense multiple access with collision detection) protocol technology. Computer network architecture refers to the convention used to define how the different protocols of the system interact with each other to support the end users.