Hughes DW4020 manual Types of hosts IANA has created three categories of hosts

Private IP addresses As Internet use has increased, more and more IP addresses have been assigned. There is not an infinite number of IP addresses, and IANA is concerned that the supply of unique IP addresses could be exhausted. One way to conserve IP addresses is to assign them only to hosts that are designated as "public." Hosts that are designated as "private" do not need unique IP addresses.

Types of hosts IANA has created three categories of hosts:

Category 1: hosts that do not require access to hosts in other enterprises or the Internet at large; hosts within this category may use IP addresses that are unambiguous within an enterprise, but may be ambiguous between enterprises. Category 2: hosts that need access to a limited set of outside services (e.g., E-mail, FTP, netnews, remote login) which can be handled by mediating gateways (e.g., application layer gateways). For many hosts in this category an unrestricted external access (provided via IP connectivity) may be unnecessary and even undesirable for privacy/security reasons. Just like hosts within the first category, such hosts may use IP addresses that are unambiguous within an enterprise, but may be ambiguous between enterprises. Category 3: hosts that need network layer access outside the enterprise (provided via IP connectivity); hosts in the last category require IP addresses that are globally unambiguous.

Hosts in categories 1 and 2 are private hosts. Hosts in Category 3 are public hosts. Hosts in categories 1 and 2 do not need to be directly accessible from other networks. These hosts might include airport arrival/departure displays, automatic teller machines, or equipment at clerical positions. Thus, these private hosts do not need unique IP address numbers. The IP addresses private hosts are assigned need only be unique within the network; they need not be directly accessible from other networks.

In addition, for security reasons, many enterprises use application layer gateways to connect their internal network to the Internet. The internal network usually does not have direct access to the Internet; thus, only one or more gateways are visible from the Internet. In this case, the internal network can use non-unique IP network numbers.

In short, there are many devices that can be connected on an internal Internet or intranet that need never be assigned unique IP addresses. In this way, IP addresses are conserved.

Private address space IANA has reserved the following three blocks of the IP address space for private internets:

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Appendix B • The Internet and the DW4020 1031484-0001 Revision 2