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ISPs

Most companies or organizations obtain their IPv4 address blocks from an Internet service provider (ISP). An ISP will generally supply a small number of usable IPv4 addresses (6 or

14)  to its customers as a part of its services. Larger blocks of addresses can be obtained based on justification of needs and at additional service costs.

In a sense, the ISP loans or rents these addresses to the organization. If you choose to move your Internet connectivity to another ISP, the new ISP will provide you with addresses from the address blocks that have been provided to it. Your previous ISP then returns the blocks loaned to your network to its allocation for loan to another customer.

ISP Services

To get access to the services of the Internet, you have to connect your data network to the Internet using an ISP. ISPs have their own set of internal data networks to manage Internet connectivity and to provide related services. Among the other services that an ISP generally provides to its customers are Domain Name System (DNS) services, e-mail services, and a website. Depending on the level of service required and available, customers use different tiers of an ISP.

ISP Tiers

ISPs are designated by a hierarchy based on their level of connectivity to the Internet back-bone. Each lower tier obtains connectivity to the backbone through a connection to a higher-tier ISP. Figure 6-12 shows the three tiers of ISPs.

Figure 6-12     Three Tiers of ISPs

Internet Backbone

Tier 1

(For Example, Sprint, Savvis)

Tier 2                                               Tier 2

(For Example, .nLayer)                      (France Telecom)

Tier 3                                               Tier 3

(For Example, Fortress ITX) (For Example, Beachcomputers