Private Addressing

Some computers will never be connected to the Internet. These computers' IP addresses could be duplicates of registered IP addresses in the Internet. So, when designing the IP addressing convention for such a network, an organization could pick and use any network number(s) that it wanted, and all would be well. For instance, you can buy a few routers, connect them together in your office, and configure IP addresses in network 1.0.0.0 and make it work. The IP addresses that you use might be duplicates of real IP addresses in the Internet, but if all you want to do is learn on the lab in your office, all is well.

When building a private network that will have no Internet connectivity, you also can use IP network numbers called private internets, as defined in RFC 1918, "Address Allocation for Private Internets" (www.ietf.org/rfc/rfc1918.txt). This RFC defines a set of networks that never will be assigned to any organization as a registered network number. Instead of using someone else's registered network numbers, you can use numbers in a range that are not used by anyone in the public Internet. Table 12-36 shows the private address space defined by RFC 1918.

Table 12-36 RFC 1918 Private Address Space

Range of IP Addresses

Class of Networks

Number of Networks

10.0.0.0 to 10.255.255.255

A

1

172.16.0.0 to 172.31.255.255

B

16

192.168.0.0 to 192.168.255.255

C

256

In other words, any organization can use these network numbers. However, no organization is allowed to advertise these networks using a routing protocol on the Internet.

Many of you might be wondering, "Why bother reseverving special private network numbers when it doesn't matter whether the addresses are duplicates?" Well, as it turns out, you can use private addressing in a network and use the Internet at the same time, as long as you use the next feature covered in this chapter—Network Address Translation (NAT).

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