Internet
Wide area networks (Internet most common).
Composed of individuals LANs of diverse technology.
Requiring a common platform independent protocols for various activity.
At connection level - TCP/IP transmission control protocol/Internet protocol
TCP/IP is the level most firewalls work at.
At user interface level, the particular task to be performed may vary
greatly :
Terminal connection (telnet, ssh).
File transfer (ftp, sftp(ftp over ssh)).
Remote document viewing (html)
Time (ntp)
Etc.
Also helpful - common symbolic labeling of "sites" to contact.