RFC 1597 (rfc1597) - Page 3 of 8

Address Allocation for Private Internets

RFC 1597        Address Allocation for Private Internets      March 1994


      - For security reasons, many enterprises use application layer
        gateways (e.g., firewalls) 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 firewall hosts are
        visible from the Internet.  In this case, the internal network
        can use non-unique IP numbers.

      - If two enterprises communicate over their own private link,
        usually only a very limited set of hosts is mutually reachable
        from the other enterprise over this link. Only those hosts need
        globally unique IP numbers.

      - Interfaces of routers on an internal network usually do not
        need to be directly accessible from outside the enterprise.

3. Private Address Space

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following three blocks of the IP address space for private networks:

        10.0.0.0        -   10.255.255.255
        172.16.0.0      -   172.31.255.255
        192.168.0.0     -   192.168.255.255

   We will refer to the first block as "24-bit block", the second as
   "20-bit block, and to the third as "16-bit" block.  Note that the
   first block is nothing but a single class A network number, while the
   second block is a set of 16 contiguous class B network numbers, and
   third block is a set of 255 contiguous class C network numbers.

   An enterprise that decides to use IP addresses out of the address
   space defined in this document can do so without any coordination
   with IANA or an Internet registry.  The address space can thus be
   used by many enterprises.  Addresses within this private address
   space will only be unique within the enterprise.

   As before, any enterprise that needs globally unique address space is
   required to obtain such addresses from an Internet registry.  An
   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.  Such hosts will be
   called private hosts, and 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 external host.  While not having external network



Rekhter, Moskowitz, Karrenberg & de Groot