Reasons for Using Specific Address Classes

---

If you are new to TCP/IP, you may be asking yourself “Why are there different classes of IP addresses, and how can I use them?” First of all, the Internet community has defined the different types of IP addresses in order to accommodate the needs of networks of different sizes. A network with less than 255 devices (workstations, routers, printers, and so) can be assigned a Class C network address. However, a large organization with up to 65,534 devices will need at least a Class B address.

Second, as long as you are not connecting your internal network directly to the public Internet, you can use any valid Class A, B, or C address you want. However, any device that is connected directly to the Internet, must be assigned a network ID from the Internet community. The organization responsible for administering the assignment of the network ID portions of IP addresses for network devices directly connected to the Internet is the Internet Network Information Center (InterNIC). They can be reached at www.internic.net.




RFC 1918 defines the methodology for IP address allocation for private networks.

For most private networks (intranets) on the border of the public Internet, IP addresses are either assigned dynamically (see Chapter 6, “Dynamic Host Configuration Protocol”) or statically by an Internet Service Provider (ISP). The ISP maintains responsibility for administering IP network IDs assigned by InterNIC. Three examples of ISPs, which dynamically assign IP addresses, commonly used by individuals for dial-up access are CompuServe, America Online, and Prodigy. Typically, a private network requiring access to the Internet will use a direct connection to an ISP through a router. In these cases, the ISP will provide a network ID to the private network. This address will be a unique statically assigned address provided to the ISP from InterNIC. These commercial services are usually provided by larger ISPs, including MCI, AT&T, and GTE.