The Internet is actually a collection of networks that are all connected together. Systems on the Internet use a communications protocol known as TCP/IP (Transmission Control Protocol/Internet Protocol). The network resources described in this article generally require access to the Internet, which allows for remote logins and file transfers. Many astronomers are connected via the NASA Science Internet (NSI), or other organizations which provide support for both TCP/IP and DECNet communications. TCP/IP numeric addresses consist of four numbers separated by decimal points (e.g., 130.167.1.2). DECNet numeric addresses are sometimes given as two numbers separated by a decimal point, or as a single number (the convention used here). A DECNet address such as 6.405 is converted to a single number by multiplying the first number by 1024 and adding the second number (6 x 1024 + 405 = 6549).