Reserved IP Address Space Reminder, (Wed, May 16th)

As we are running out of IPv4 address space, many networks, instead of embracing IPv6, stretch existing IPv4 space via multiple levels of NAT. NAT then uses reserved IP address space. However, there are more address ranges reserved then listed in RFC1918, and not all of them should be used in internal networks. Here is a (probably incomplete) list of address ranges that are reserved, and which once are usable inside your network behind a NAT gateway.

List of Reserved IPv4 Address ranges


Address Range
RFC
Suitable for Internal Network




0.0.0.0/8
RFC1122
no (any address)


10.0.0.0/8
RFC1918
yes


100.64.0.0/10
RFC6598
yes (with caution: If you are a carrier)


127.0.0.0/8
RFC1122
no (localhost)


169.254.0.0/16
RFC3927
yes (with caution: zero configuration)


172.16.0.0/12
RFC1918
yes


192.0.0.0/24
RFC5736
no (not used now, may be used later)


192.0.2.0/24
RFC5737
yes (with caution: for use in examples)


192.88.99.0/24
RFC3068
no (6-to-4 anycast)


192.168.0.0/16
RFC1918
yes


198.18.0.0/15
RFC2544
yes (with caution: for use in benchmark tests)


198.51.100.0/24
RFC5737
yes (with caution: test-net used in examples)


203.0.113.0/24
RFC5737
yes (with caution: test-net used in examples)


224.0.0.0/4
RFC3171
no (Multicast)


240.0.0.0/4
RFC1700
no (or unwise? reserved for future use)



Most interesting in this context is RFC6598 (100.64.0.0/10), which was recently assigned to provide ISPs with a range for NAT that is not going to conflict with their customers NAT networks. It has been a more and more common problem that NAT'ed networks once connected with each other via for example a VPN tunnel, have conflicting assignments.
Which networks did I forget? I will update the table for a couple days as comments come in.
------

Johannes B. Ullrich, Ph.D.

SANS Technology Institute

Twitter (c) SANS Internet Storm Center. http://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.