Internet service provider (ISP)

An Internet service provider (ISP) is an organization that provides access to the Internet. Access ISPs directly connect clients to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs are a kind of colocation centre that leases server space to smaller businesses and other people. Transit ISPs provide large amounts of bandwidth for connecting hosting ISPs to access ISPs.

Internet service providers may be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privately owned.

Access providers

ISPs employ a range of technologies to enable consumers to connect to their network. If users and small businesses, traditional options include: dial-up, DSL (typically Asymmetric Digital Subscriber Line, ADSL), broadband wireless, cable modem, fiber to the premises (FTTH), and Integrated Services Digital Network (ISDN) (typically basic rate interface).

For customers with more demanding requirements, such as medium-to-large businesses, or other ISPs, DSL (often Single-Pair High-speed Digital Subscriber Line or ADSL), Ethernet, Metropolitan Ethernet, Gigabit Ethernet, Frame Relay, ISDN (B.R.I. or P.R.I.), ATM (Asynchronous Transfer Mode) and upload satellite Internet access. Sync-optical cabling (SONET) is more likely to be used.

Many access providers also provide hosting and email services.

Mailbox providers

A mailbox provider is a department or organization that provides email mailbox hosting services. It provides email servers to send, receive, accept, and store email for other organizations and/or end users, on their behalf and upon their explicit mandate.

Many mailbox providers are also access providers, while others aren't (e.g., Yahoo! Mail, Hotmail, Gmail, AOL Mail, Pobox). The definition given in RFC 6650 covers email hosting services, as well as the relevant department of companies, universities, organizations, groups, and individuals that manage their mail servers themselves. The task is typically accomplished by implementing Simple Mail Transfer Protocol (SMTP) and possibly providing access to messages through Internet Message Access Protocol (IMAP), the Post Office Protocol, Webmail, or a proprietary protocol.

Hosting ISPs

Hosting ISPs routinely provide email, FTP, and web-hosting services. Other services include virtual machines, clouds, or entire physical servers where customers can run their own custom software.

Transit ISPs

Just as their customers pay them for Internet access, ISPs themselves pay upstream ISPs for Internet access. An upstream ISP usually has a larger network than the contracting ISP and/or is able to provide the contracting ISP with access to parts of the Internet the contracting ISP by itself has no access to.

In the simplest case, a single connection is established to an upstream ISP and is used to transmit data to or from areas of the Internet beyond the home network; this mode of interconnection is often cascaded multiple times until reaching a Tier 1 carrier. In reality, the situation is often more complex. ISPs with more than one point of presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and may have connections to each one of them at one or more point of presence.

Virtual ISPs

A Virtual ISP (VISP) is an operation which purchases services from another ISP (sometimes called a "wholesale ISP" in this context) which allow the VISP's customers to access the Internet using services and infrastructure owned and operated by the wholesale ISP.

Free ISPs

Free ISPs are Internet Service Providers (ISPs) which provide service free of charge. Many free ISPs display advertisements while the user is connected; like commercial television, in a sense they are selling the users' attention to the advertiser. Other free ISPs, often called freenets, are run on a nonprofit basis, usually with volunteer staff.