An ISP is either an institution or a paid-for service provider that literally gives its clients access to the Internet. An ISP will typically buy a block of IP addresses from an higher level ISP (e.g. for UK academia that is JANET which runs the ac.uk domain) which it can allocate to its customers and name as it wishes. ISPs are usually associated with a domain name, e.g. swan.ac.uk, and to be good citizens on the Internet, must provide the domain name service (DNS) and authoritative naming for the fully qualified domain names (FQDN) of all the hosts (IP addresses) that the ISP has ownership of.

In modern systems, ISPs allocated their supply of IP addresses to hosts on-demand using a protocol known as DHCP.

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