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




 
RFC 1918        Address Allocation for Private Internets   February 1996


                  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.

   We will refer to the hosts in the first and second categories as
   "private".  We will refer to the hosts in the third category as
   "public".

   Many applications require connectivity only within one enterprise and
   do not need external (outside the enterprise) connectivity for the
   majority of internal hosts. In larger enterprises it is often easy to
   identify a substantial number of hosts using TCP/IP that do not need
   network layer connectivity outside the enterprise.

   Some examples, where external connectivity might not be required,
   are:

         - A large airport which has its arrival/departure displays
           individually addressable via TCP/IP. It is very unlikely
           that these displays need to be directly accessible from
           other networks.

         - Large organizations like banks and retail chains are
           switching to TCP/IP for their internal communication. Large
           numbers of local workstations like cash registers, money
           machines, and equipment at clerical positions rarely need
           to have such connectivity.

         - 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.

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







 
RFC 1918        Address Allocation for Private Internets   February 1996


3. Private Address Space

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

     10.0.0.0        -   10.255.255.255  (10/8 prefix)
     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

   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 (in
   pre-CIDR notation) 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 256 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, or the set of enterprises which
   choose to cooperate over this space so they may communicate with each
   other in their own private internet.

   As before, any enterprise that needs globally unique address space is
   required to obtain such addresses from an Internet registry. An