 ISPs- -These ISPs have national coverage. They include the incumbent telecom carriers and the new competitive entrants. Regional ISPs-These ISPs are active regionally throughout a nation or world zone, and they own their equipment and lease their lines from the incumbent telco or competitive operator. Retail ISPs-These ISPs have no investment in the network infrastructure whatsoever. They basically use their brand name and outsource all the infrastructure to an NSP or a high-tier ISP, but they build from a known customer database that is loyal and provides an opportunity to offer a branded ISP service. These various levels of NSPs interconnect, and they can connect in several ways. In the early days of the Internet, there were four main network access points (NAPs), all located in the United States, through which all of the world's Internet traffic interconnected and traveled. The four original NAPs were located in San Francisco (PacBell NAP, operated by PacBell), Chicago (AADS, operated by Ameritech), Tysons Corner, Virginia (MAE-East, operated by MCI Worldcom), and Pennsauken, New Jersey (SprintNAP, operated by Sprint) These IXPs are also called metropolitan area exchanges (MAEs). Today there are hundreds of IXPs distributed around the world. An IXP is a physical infrastructure where different NSPs and/or ISPs can exchange traffic between their networks with their counterparts. By means of mutual peering arrangements, traffic can be exchanged without cost. An IXP is therefore a public meeting point that houses cables, routers, switches, LANs, data communications equipment, and network management and telecommunications links. IXPs allow the NSPs and top-tier ISPs to exchange Internet traffic without having to send the traffic through a main transit link. This translates to decreased costs associated with the transit links, and it reduces congestion on the transit links. The main purpose of an IXP is to allow providers to interconnect via the exchange rather than having to pass traffic through third-party networ