"But who is my ISP?"
Your ISP is the company you pay for Internet access. If you have high-speed access such as cable modem or DSL, then your ISP is probably your cable company or phone company, although there are other companies that offer these services too (notably Earthlink). If you have old-fashioned, slow "dialup" access, you could be with AOL, Earthlink or any number of smaller ISPs.
If you are the person who pays the bills in your house, you'll already know. If not, just ask that person.
"But I'm not paying anybody!"
If you have a computer with wireless Internet access (WiFi), there's a possibility that you are "borrowing" Internet access from a neighbor with an unsecured WiFi access point. In that case, you are stealing Internet service from someone else, who is acting as your ISP without your permission. Be aware that this is a criminal act in the United States and individuals who do it have recently been charged with data trespassing, a federal crime. If you are poor and wish to obtain cheap Internet access, I suggest that you talk to your neighbor and offer to pay a portion of their Internet bill in exchange for permission to use their WiFi connection. While you're at it, teach them how to secure their WiFi correctly with WPA so that only the two of you have access. A WiFi network that you can sneak onto is also a WiFi network that others can monitor. That means that someone could be listening to all of your Internet activities on an unsecured WiFi network (except for SSL-encrypted connections to secure sites and other encrypted traffic).
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