An internet is a network of networks. The term was coined for the ARPANET to describe a collection of different networks that all used TCP/IP. IP 'datagrams' (data entities) could be transported, encapsulated in a wide variety of different ways, within a certain network, such as a local Ethernet, the ARPANET or the satellite network SATNET, but an IP packet was passed unchanged from sender to recipient. IP networks could be compared to automated container deliveries, direct from the factory conveyor belt right to the customer.

Before the commercialisation of the Internet, 'Internet' with a capital 'I' meant the science community's worldwide computer network that used internet technology. While 'internet' written with a small 'i' referred to any network that used the same technology, whether it was connected to the major Internet or not. It is hard to judge the scale of a network from the way its name is written, and so a distinction is now being made between the worldwide Internet and closed internets within organisations. The latter are called 'Intranets', while internets internal to a certain group of organisations are called 'Extranets'. For instance, Extranets have been created for transmitting data between major car factories and their subcontractors. Most of these closed internets still have at least WWW and e-mail links with the outside world via 'firewalls', which act as the company's Internet gatekeepers.