s explained earlier, the IPv4's 32-bit address field sets a theoretical maximum number of addresses to 232 = 4,294,967,296 addresses. To understand the address space usage more clearly, we can consider the IPv4 address space to consist of 256 address blocks of the size of /8. As described earlier, we have address space that cannot be used for normal communication purposes in the internet. Including all the different special use address spaces in RFC 5735 [11], we have 35.078 /8s (14%) that cannot be used on the Internet. This leaves 220.922 /8s, which can be used for Internet nodes - a range of 3,706,456,113 addresses - just under four billion. As all nodes that are connected to the Internet need their own unique address, just under four billion individual nodes can be at the same time on the Internet. These nodes include the routers, servers, end-hosts - basically all nodes connected to the Internet, which have to be reachable from the Internet. At the time writing, the world population is around 7 billion, and there are already over two billion Internet users, and the number is growing Deploying IPv6 in 3GPP Networks fast. Introduction of the Internet of Things (see Section 6.5) creates significant additional pressure on the address space consumption. It is clear that the IPv4 address space is a seriously constrained resource. To clearly understand the IP address allocation problem, and the IPv4 address space exhaustion, we will now look at how IP addresses are allocated to the end users, what is the history of IPv4 address space exhaustion, and what has been done to mitigate the depletion of the IPv4 address space. Finally, we will look at the situation at the time of writing this book. 1.7.1 IP Address Allocation The IP address allocation is a hierarchical system. Figure 1.5 shows how the IP address allocation hierarchy is set up. This is one representation of the hierarchy. You can certainly find other descriptions where the boxes are in a different order. The IP address allocation should be more of a techni