The Internet is composed of many networks and/or subnets
interconnected by routers.
Where the networks and/or subnets to be connected are not
geographically adjacent, it is common for a permanent
point-to-point link to be used. The historical
terminology for a full-time telecommunications interconnect of this
kind is a "leased line"[1]. The name derives from the ancient
technology of renting a permanently connected "phone line" plus the
necessary modems at each end.
Historically, a point-to-point inter-router link was normally a
separate subnet -- one with only two IP addresses
allocated.
Organisations which wish to establish full-time links to the
Internet for their internal networks need to establish a
router-to-router link to another router which has Internet access.
This can be done "cooperatively", where one
Internet-connected organisation allows another to establish a link
to its network, but nowadays is most commonly done by dealing
directly with an Internet Service Provider
(ISP).
[1] The traditional
"leased line" provides a so-called basic carriage
service. Such services are not, in general, very
profitable for a telecommunications company.