enterprise that requests IP addresses for its external connectivity
   will never be assigned addresses from the blocks defined above.

   In order to use private address space, an enterprise needs to
   determine which hosts do not need to have network layer connectivity
   outside the enterprise in the foreseeable future and thus could be
   classified as private. Such hosts will use the private address space
   defined above.  Private hosts can communicate with all other hosts
   inside the enterprise, both public and private. However, they cannot
   have IP connectivity to any host outside of the enterprise. While not
   having external (outside of the enterprise) IP connectivity private
   hosts can still have access to external services via mediating
   gateways (e.g., application layer gateways).

   All other hosts will be public and will use globally unique address
   space assigned by an Internet Registry. Public hosts can communicate
   with other hosts inside the enterprise both public and private and
   can have IP connectivity to public hosts outside the enterprise.
   Public hosts do not have connectivity to private hosts of other
   enterprises.






 
RFC 1918        Address Allocation for Private Internets   February 1996


   Moving a host from private to public or vice versa involves a change
   of IP address, changes to the appropriate DNS entries, and changes to
   configuration files on other hosts that reference the host by IP
   address.

   Because private addresses have no global meaning, routing information
   about private networks shall not be propagated on inter-enterprise
   links, and packets with private source or destination addresses
   should not be forwarded across such links. Routers in networks not
   using private address space, especially those of Internet service
   providers, are expected to be configured to reject (filter out)
   routing information about private networks. If such a router receives
   such information the rejection shall not be treated as a routing
   protocol error.

   Indirect references to such addresses should be contained within the
   enterprise. Prominent examples of such references are DNS Resource
   Records and other information referring to internal private
   addresses. In particular, Internet service providers should take
   measures to prevent such leakage.

4. Advantages and Disadvantages of Using Private Address Space

   The obvious advantage of using private address space for the Internet
   at large is to conserve the globally unique address space by not
   using it where global uniqueness is not required.

   Enterprises themselves also enjoy a number of benefits from their
   usage of private address space: They gain a lot of flexibility in
   network design by having more address space at their disposal than
   they could obtain from the globally unique pool. This enables
   operationally and administratively convenient addressing schemes as
   well as easier growth paths.

   For a variety of reasons the Internet has already encountered
   situations where an enterprise that has not been connected to the
   Internet had used IP address space for its hosts without getting this
   space assigned from the IANA. In some cases this address space had
   been already assigned to other enterprises. If such an enterprise
   would later connects to the Internet, this could potentially create
   very serious problems, as IP routing cannot provide correct
   operations in presence of ambiguous addressing. Although in principle
   Internet Service Providers should guard against such mistakes through
   the use of route filters, this does not always happen in practice.
   Using private address space provides a safe choice for such
   enterprises, avoiding clashes once outside connectivity is needed.






 
RFC 1918        Address Allocation for Private Internets   February 1996


   A major drawback to the use of private address space is that it may
   actually reduce an enterprise's flexibility to access the Internet.
   Once one commits to using a private address, one is committing to
   renumber part or all of an enterprise, should one decide to provide
   IP connectivity between that part (or all of the enterprise) and the
   Internet.  Usually the cost of renumbering can be measured by
   counting the number of hosts that have to transition from private to
   public. As was discussed earlier, however, even if a network uses
   globally unique addresses, it may still have to renumber in order to
   acquire Internet-wide IP connectivity.

   Another drawback to the use of private address space is that it may
   require renumbering when merging several private internets into a
   single private internet. If we review the examples we list in Section