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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP "Classful" (Conventional) Addressing

IP Addresses With Special Meanings 1 2 3 IP Multicast Addressing

Private/Unregistered/Non-Routable Addresses

Recall that in the IP address overview I contrasted private and public IP addresses. Every IP address on a IP network must be unique, and in the case of a public IP network, addresses are allocated using a central authority to ensure that there is no overlap. In contrast, if you set up a private network you can use whatever addresses you want; it's your show and nobody else has a say on what you do.

So, if this is the case, why not just pick any random block of class A, B or C addresses for your private network and use that? Well, you could; since you aren't connected to the Internet you could use, say, the class A network 18.x.x.x that is reserved on the Internet to the Massachusetts Institute of Technology (MIT). Since you aren't connected to MIT, what does it matter?

In the past, there were people who did choose to do this—and ended up regretting that decision. As the Internet grew in importance, they would later decide that such private networks needed to connect to the public Internet after all. If that happened and you had used 18.x.x.x addresses, you'd find yourself having to renumber all your devices to avoid getting a big bunch of geeks really angry. (There were in fact cases where companies that had used IP address space belonging to other companies accidentally connected those machines to the Internet, causing a small amount of ruckus in the process.)

From a more mundane standpoint, having internal devices with MIT's IP addresses is a source of confusion. It's just not a great idea to have lots of different companies “borrowing” IP addresses for their private networks.

As an alternative, RFC 1918 (superseding RFC 1597) defines a set of special address blocks that are set aside just for private addresses. These addresses simply don't exist to the public Internet. Anyone can use them with no need to contact any authority for permission. At the same time, they cannot connect to the global Internet, because routers are not programmed with entries to forward traffic with these address ranges outside of local organizations. RFC 1918 was in fact published to encourage the use of these private blocks, in order to cut down on the number of devices on the public Internet that didn't really need to be publicly-accessible. This was in response to the need to conserve the public address space.

In order to connect a network using private addressing to the public Internet, it is necessary to employ additional hardware and/or software. A gateway machine can be used as an interface between the public and private networks. Technologies such as IP Network Address Translation (NAT) are often used in conjunction with private IP addresses to allow these hosts to communicate on the public IP network.

IP Addresses With Special Meanings 1 2 3 IP Multicast Addressing