Lesson 1

Determining IP Addressing Requirements

2-7

Using Unregistered Addresses

Most TCP/IP networks use unregistered IP addresses (also called private network

addresses) for the servers and workstations that only internal users need to access.

These are addresses that are not registered with the IANA, and as a result, they are

invisible to the Internet. Because they are invisible, Internet criminals cannot specifi-

cally target them for virus distribution or other types of compromise (although they are

still vulnerable in other ways). As described in RFC 1918, “Address Allocation for Pri-

vate Internets,” the IANA has set aside three IP address ranges for use by private net-

works. These addresses are not registered to any single network, so anyone can use

them for computers and other devices on a private network.

The private IP address ranges designated by the IANA are as follows:

■

10.0.0.0 through 10.255.255.255

■

172.16.0.0 through 172.31.255.255

■

192.168.0.0 through 192.168.255.255

Tip

On a private network that is not connected to the Internet in any way, you can use any IP

addresses you want to, registered or not, because there is no way for them to conflict with

the registered users of those addresses on the Internet. However, if your network users

access the Internet in any way, you should always use the designated private address ranges

to prevent conflicts with Internet computers.

Accessing the Internet from a Private Network

The logical question that remains, however, when you elect to use unregistered IP

addresses on your network, is how your users can access the Internet. If unregistered

addresses are invisible to the Internet, how is an Internet Web server supposed to

respond to a request from a browser on an unregistered network? The answer is that

the network designer incorporates a mechanism into the network infrastructure that

enables unregistered clients to access Internet services. The two most common mech-

anisms of this type are NAT and proxy servers.

Using Network Address Translation

Network address translation is an application built into a router that functions as an

intermediary between unregistered clients on a private network and registered Internet

servers. Client computers can use NAT to send requests to Internet servers and receive

replies, despite the fact that the clients have unregistered network addresses. This pro-

vides the unregistered computers with Internet access, without compromising their

protection from Internet intrusion.