Lesson 1

Determining IP Addressing Requirements

2-5

Why Use Registered Addresses? If you have computers on your network that you

want to be accessible from the Internet (such as Web servers), you must configure

them with IP addresses that the IANA has registered. This is because only registered

addresses are visible from the Internet. For a user on the Internet to access your com-

pany Web server, a client application, such as a Web browser, must initiate communi-

cation by sending a request to the server. The browser can’t do that if it doesn’t have

the server’s address. (Users on your network who want to access Internet services do

not require registered addresses; this matter is covered later in this lesson.)

Why Not Use Registered Addresses? Theoretically, you can use registered IP addresses

for all the computers on your network, but this practice has two serious drawbacks:

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It depletes the IP address space. If every device with an IP address today (which

includes a great many mobile telephones, automobiles, and other devices, in addi-

tion to computers) had a registered IP address, the pool of available addresses

would be well on its way to depletion. Even now, a program to expand the IP

address space from 32 (called Internet Protocol Version 4 or IPv4) to 128 bits,

called IPv6, is currently under way to prevent the possibility of depleting the

entire IP address space in the future.

See Also

For more information about IPv6, see Understanding IPv6 (Microsoft Press,

2003). Additionally, the Internet Engineering Task Force (IETF) has published a number of pro-

posed Requests for Comments (RFC) standards that you can consult, such as RFC 2464,

“Transmission of IPv6 Packets over Ethernet Networks.”

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Using registered IP addresses on a private network presents a serious security haz-

ard. Not only can a computer with a registered IP address access systems on the

Internet, the systems on the Internet can also access the computer.

Security Alert

You must set up some sort of firewall to protect Web servers and other com-

puters that must have registered addresses. For example, you can use packet filtering to permit

only Hypertext Transfer Protocol (HTTP) traffic using port 80 to reach your Web server from the

Internet. This means that Internet users can access the Web server using only standard browser

requests. Other types of traffic—such as those used by Internet predators to plant viruses, steal

data, and cause mayhem—are blocked. Without some protection, an intruder will eventually target

a registered system, and the results can range from irritating to catastrophic.

Protecting computers with registered addresses is a complex process that requires con-

stant vigilance from the network’s administrators. If you configure all your computers

with registered addresses, you compound this protection process unnecessarily. You

can use several methods to assign unregistered IP addresses to your network’s comput-

ers while still enabling them to access the Internet.