Stands for "Internet Service Provider." An ISP provides access to the Internet. Whether you're at home or work, each time you connect to the Internet, your connection is routed through an ISP.
Early ISPs provided Internet access through dial-up modems. This type of connection took place over regular phone lines and was limited to 56 Kbps. In the late 1990s, ISPs began offering faster broadband Internet access via DSL and cable modems. Some ISPs now offer high-speed fiber connections, which provide Internet access through fiber optic cables. Companies like Comcast and Cox provide cable connections, while companies like AT&T and Verizon provide DSL Internet access.
To connect to an ISP, you need a modem and an active account. When you connect a modem to the telephone or cable outlet in your house, it communicates with your ISP. The ISP verifies your account and assigns your modem an IP address. Once you have an IP address, you are connected to the Internet. You can use a router (which may be a separate device or built into the modem) to connect multiple devices to the Internet. Since each device is routed through the same modem, they will all share the same public IP address assigned by the ISP.
ISPs act as hubs on the Internet since they are often connected directly to the Internet backbone. Because of the large amount of traffic ISPs handle, they require high bandwidth connections to the Internet. In order to offer faster speeds to customers, ISPs must add more bandwidth to their backbone connection in order to prevent bottlenecks. This can be done by upgrading existing lines or adding new ones.