RFC 1597 (rfc1597) - Page 3 of 8
Address Allocation for Private Internets
Alternative Format: Original Text Document
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