The internet is a hierarchical structure linking different levels of service providers whose millions of devices supply all the interconnections. the three levels outlined in the book from the top down include

National service provider (NSP)

Regional service provider (RSP)

Internet service provider (ISP)

Explanation:

National service providers (NSP)  are companies that own the internet backbone infrastructure which other second party internet service providers can link to. Examples of NSPs are Orange, Sprint,  AT&T etc.  

It typically provides fibre optic cables and core routers which the ISPs link to in order to provide internet exchange for the customers.

Regional service provider (RSP)  are basically ISPs operating within a region. Unlike ISPs, they cover only defined regions. Examples are New England's NEARNet which provides internet access for residents of New England and the San Francisco Bay area BARNet for San Francisco Bay resident.

Internet service provider (ISP) are the direct link to the customers. Many NSPs also act as ISPs by using routers that can transfer network from the backbone network exchange to the receiving equipment of the end users such as mobile phones, computers etc. Examples of ISPs are AT&T, Comcast and Verizon.

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