Internet Service Provider
An internet service provider manages connectivity using Internet Protocol version 4 (IPv4), Internet Protocol version 6 (IPv6), or both. It creates connectivity, as defined by the terms and conditions of the business agreement, among the customers and servers inside a single enterprise (i.e., one or more intranets, depending on connectivity policies), among the endpoints of a predefined set of multiple enterprises (i.e., one or more extranets), from an ISP customer workstation to arbitrary public servers on the Internet, and possibly from arbitrary public Internet users to servers operated for or by the customer enterprise.
Small to medium ISPs connect to one or (preferably) more "upstream" IP providers that have higher-capacity Internet links and sell transit into the public Internet. Other than the smallest ISPs, they will interconnect to upstreams and to peers use the Border Gateway Protocol (BGP). ISPs that run BGP will normally have their own Autonomous System number.
When an ISP connects to more than one upstream, such that it does not have a single point of failure for Internet access, it is said to be multihomed. ISPs may offer many forms of multihoming, fault tolerance, and disaster recovery to their customers. The ISP may provide the customer with separate physical paths to geographically separate points of presence of the ISP, or may contract to manage the customer's connectivity to additional ISPs that do not share common points of failure with the main ISP.
ISPs have various ways to provide end user connectivity to their customers, ranging from low-speed modem dialup on the Public Switched Telephone Network (PSTN), to data over cellular telephones, to metropolitan-area wireless local area networks (WLAN), to data over cable television systems or digital subscriber loop (DSL) with wire leased from a telephone company, to direct high-speed connections over optical fiber.
ISPs "lease" or "dynamically assign" IP addresses to their smaller customers who cannot justify their own block of addresses. Depending on the technical capabilities of the customer, some combination of customer and ISP will manage the own "forward" Domain Name Service (DNS) servers[1] and "reverse" DNS [2]. They may operate certain common Internet application infrastructure tools, such as Simple Mail Transfer Protocol (SMTP) email servers, World Wide Web performance accelerators such as web caches, and Network News Transfer Protocol (NNTP) servers for "netnews"/USENET.
Depending on their business niche, ISPs provide varying levels of user support to customers, ranging from being responsible only for basic IP connectivity, to extensive support with end user applications and perhaps the end user operating system. ISPs often differentiate for residential, small and home office, and large enterprise segments.