These are specified in RFC1597 and are as follows:

Reserved Private Network Allocations
Network Class  Netmask  Network Address 
A -
B -
C -

"Auto configuration" IP Addresses: -

Addresses in the range to are used automatically by some PCs and Macs 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 3330.

"Loopback" IP addresses: -

Each computer on the Internet uses to identify itself, to itself. to is earmarked for what is called "loopback". This construct allows a computer to establish/validate its IP stack. Most software only uses 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 bootstrapping. This is documented in RFC 3330.

"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: -

Addresses in the range to 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.) 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.