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Tier 1

At the top of the ISP hierarchy are tier 1 ISPs. These ISPs are large national or international ISPs directly connected to the Internet backbone. The customers of tier 1 ISPs are either lower-tiered ISPs or large companies and organizations. Because they are at the top of Internet connectivity, they engineer highly reliable connections and services. Among the technologies used to support this reliability are multiple connections to the Internet backbone.

The primary advantages for customers of tier 1 ISPs are reliability and speed. Because these customers are only one connection away from the Internet, there are fewer opportunities for failures or traffic bottlenecks. The drawback for tier 1 ISP customers is the high cost.

Tier 2

Tier 2 ISPs acquire their Internet service from tier 1 ISPs. Tier 2 ISPs generally focus on business customers and usually offer more services than the other two tiers of ISPs. Tier 2 ISPs tend to have the IT resources to operate their own services such as DNS, e-mail servers, and web servers. Other services that tier 2 ISPs can offer include website develop-ment and maintenance, e-commerce/e-business, and VoIP.

The primary disadvantage of tier 2 ISPs, as compared to tier 1 ISPs, is slower Internet access. Because tier 2 ISPs are at least one more connection away from the Internet back-bone, they also tend to have poorer reliability than tier 1 ISPs.

Tier 3

Tier 3 ISPs purchase their Internet service from tier 2 ISPs. The focus of these ISPs is the retail and home markets in a specific locale. Tier 3 customers typically do not need many of the services required by tier 2 customers. Their primary need is connectivity and support.

These customers often have little or no computer or network expertise. Tier 3 ISPs often bundle Internet connectivity as a part of network and computer service contracts for their customers. Although they might have reduced bandwidth and less reliability than tier 1 and tier 2 providers do, they are often good choices for small- to medium-size companies.

Calculating Addresses

To work with IPv4 networks, you need to be able to develop and determine proper address-ing. These skills include the ability to determine whether a particular host is on a network, determine the addresses in a particular network, and determine how to divide an addressing scheme for an internetwork.

In the following sections, you will be presented with techniques to make these determina-tions. There will also be several examples demonstrating how these are accomplished.