1. What is the current design of the Internet?

As noted earlier and illustrated in Figure 3.6, the Internet's infrastructure comprises numerous backbones operated by national, regional, and local Internet service providers. The major national backbones are high-speed networks owned by telecommunications organizations such as Sprint, AT&T, and WorldCom as well as by major cable companies such as TimeWarner, which recently merged with America Online (AOL), and satellite providers. Regional backbones are generally owned by regional Bell operating companies (RBOCs) such as BellSouth and Verizon (formerly Bell Atlantic) or by newly formed competitive local exchange carriers (CLECs). Local backbones are typically operated by small, independent Internet service providers (ISPs) or by CLECs. The major backbone providers (sometimes called Tier 1 ISPs) provide access to regional and local ISPs; many regional providers also provide access to local ISPs.

1. Are all of these backbones independent of one another? If so, then how do my e-mail messages get to their correct destinations if I use a different provider (and hence, a different backbone) than the recipients of my messages?

The national backbones are indeed pretty much independent of one another. Given the multitude of independent backbones, the Internet relies on network access points (NAPs) and a routing arbiter for interregional connectivity. Network access points are special high-speed Internet exchange points (IXPs) or "switching stations" where different network backbone providers meet and exchange traffic on each other's backbone. This is where the individual networks comprising the Internet join together. NAPs...