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 between 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.  When such an enterprise
   later connects to the Internet, it could potentially create very




 
RFC 1597        Address Allocation for Private Internets      March 1994


   serious problems, as IP routing cannot provide correct operations in
   presence of ambiguous addressing.  Using private address space
   provides a safe choice for such enterprises, avoiding clashes once
   outside connectivity is needed.

   One could argue that the potential need for renumbering represents a
   significant drawback of using the addresses out of the block
   allocated for private internets.  However, we need to observe that
   the need is only "potential", since many hosts may never move into
   the third category, and an enterprise may never decide to
   interconnect (at IP level) with another enterprise.

   But even if renumbering has to happen, we have to observe that with
   Classless Inter-Domain Routing (CIDR) an enterprise that is connected
   to the Internet may be encouraged to renumber its public hosts, as it
   changes its Network Service Providers.  Thus renumbering is likely to
   happen more often in the future, regardless of whether an enterprise
   does or does not use the addresses out of the block allocated for
   private networks.  Tools to facilitate renumbering (e.g., DHCP) would
   certainly make it less of a concern.

   Also observe that the clear division of public and private hosts and
   the resulting need to renumber makes uncontrolled outside
   connectivity more difficult, so to some extend the need to renumber
   could be viewed as an advantage.

5. Operational Considerations

   A recommended strategy is to design the private part of the network
   first and use private address space for all internal links.  Then
   plan public subnets at the locations needed and design the external
   connectivity.

   This design is not fixed permanently.  If a number of hosts require
   to change status later this can be accomplished by renumbering only
   the hosts involved and installing another physical subnet if
   required.

   If a suitable subnetting scheme can be designed and is supported by
   the equipment concerned, it is advisable to use the 24-bit block of
   private address space and make an addressing plan with a good growth
   path.  If subnetting is a problem, the 16-bit class C block, which
   consists of 255 contiguous class C network numbers, can be used.

   Using multiple IP (sub)nets on the same physical medium has many
   pitfalls. We recommend to avoid it unless the operational problems
   are well understood and it is proven that all equipment supports this
   properly.




 
RFC 1597        Address Allocation for Private Internets      March 1994


   Moving a single host between private and public status will involve a
   change of address and in most cases physical connectivity.  In
   locations where such changes can be foreseen (machine rooms etc.)  it
   may be advisable to configure separate physical media for public and
   private subnets to facilitate such changes.

   Changing the status of all hosts on a whole (sub)network can be done
   easily and without disruption for the enterprise network as a whole.
   Consequently it is advisable to group hosts whose connectivity needs
   might undergo similar changes in the future on their own subnets.

   It is strongly recommended that routers which connect enterprises to
   external networks are set up with appropriate packet and routing
   filters at both ends of the link in order to prevent packet and