Several address ranges are reserved for "Special Use". These addresses all have restrictions of some sort placed on their use, and in general should not appear in normal use on the public Internet. The overview below briefly explains the purpose of these addresses – in general they are used in specialized technical contexts. They are described in more detail in RFC 6890.
- "Private Use" IP addresses:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255
These address blocks are reserved for use on private networks, and should never appear in the public Internet. There are millions of private networks (for example home firewalls often use them). People can use these address blocks without informing us, so we have no record of who uses which of these addresses.
The point of private address space is to allow many organizations in different places to use the same addresses, and as long as these disconnected or self-contained islands of IP-speaking computers (private networks) are not connected, there is no problem. If you see an apparent attack, or spam, coming from one of these address ranges, then either it is coming from your local environment, your ISP, or the address has been "spoofed".
The Private addresses are documented in RFC 1918. If you
have further questions about RFC 1918 usage, please contact your
ISP.
- "Autoconfiguration" IP Addresses:
169.254.0.0 - 169.254.255.255
Addresses in the range 169.254.0.0 to 169.254.255.255 are used automatically by most network devices when they are configured to use IP, do not have a static IP Address assigned and are unable to obtain an IP address using DHCP.
This traffic is intended to be confined to the local network,
so the administrator of the local network should look for misconfigured
hosts. Some ISPs inadvertently also permit this traffic, so you
may also want to contact your ISP. This is documented in RFC 6890.
- "Loopback" IP addresses:
127.0.0.0 - 127.255.255.255
Each computer on the Internet uses 127.0.0.0/8 to identify itself, to itself. 127.0.0.0 to 127.255.255.255 is earmarked for what is called "loopback". This construct allows a computer to confirm that it can use IP and for different programs running on the same machine to communicate with each other using IP. Most software only uses 127.0.0.1 for loopback purposes (the other addresses in this range are seldom used). All of the addresses within the loopback address are treated with the same levels of restriction in Internet routing, so it is difficult to use any other addresses within this block for anything other than node specific applications, generally bootstraping. This is documented in RFC 6890.
- "Unallocated" IP addresses:
The IPv4 Address Registry
and the Whois use the word unallocated (sometimes "reserved")
to mean that the addresses are reserved for future allocation.
No one should be using these addresses now. These addresses will
be assigned for use in the public Internet in the future. If addresses
are needed for private networks then the private-use addresses
mentioned above should be used.
- Multicast IP addresses:
224.0.0.0 - 239.255.255.255
Addresses in the range 224.0.0.0 to 239.255.255.255 are set aside for the special purpose of providing multicast services in the Internet. Multicast services allow a computer to send a single message to many destinations. Examples include the software that keeps computers’ clocks synchronised and television services delivered over IP, typically by cable ISPs. Various addresses in this range are used by routers and others are used by hosts that are listening to multicast sessions.
These addresses are available for any host that wants to participate
in multicast, and typically are assigned dynamically. The source
address should not be multicast (without prior agreement). The
destination address may be multicast. For technical background
information please see RFC 1112 and RFC 2236.