s or globally routable addresses. In addition, other address types also exist. The specification describing special use IPv4 addresses [11] lists 15 different special address blocks. We will here go through the most important ones. As will be seen below, the IPv4 addresses are mainly used for the unicast communication between network nodes. In addition, however, there are special addresses with special meanings. Most of the IPv4 address space's 4 billion addresses are used for normal unicast addresses for the communication over the Internet, but there is quite a bit address space that cannot be used for that purpose. This restricts the number of addresses in the Internet even further. We will discuss how IPv4 address scarcity is an issue later on. Private Use Networks 10/8, 172 16/12 and 192 .168/16 are reserved for private use networks [12]. Commonly this part of the address space is called private address space, and the addresses private addresses. These addresses can be used only within a private network. They can- not be used directly in the Internet, because the addresses given from a private address block are not unique over the whole Internet. Shared Address Space 100.64/10 is reserved for shared address space [13]. Shared address space is similar to private addresses introduced above, but instead of general use, the shared address space is intended to be used within an operator network. Introduction Loopback 127.0.0.0/8 is reserved to loopback addresses. These addresses are exclusively used for host internal communication. Packets with loopback addresses should not be seen on any network. Link-local Addresses 169.254/16 address block is used for communication constrained to one link. These addresses are especially used before a host has gotten a real address, private or public. In some cases, such as with directly connected hosts, these may be the only addresses ever available on a link RFC 3927 [44]. Multicast 224.0.0.0/4 block is used for multicast services. How the multicast address space is used is sp