An IPv4 address has 32 bits and looks familiar. An IPv6 address has 128 bits and looks wild at first glance. Extending the address space was one of the driving reasons to develop IPv6, along with optimization of routing tables, especially on the Internet. This chapter will help you become familiar with the extended address space and will also explain how IPv6 addressing works and why it has been designed to be the way it is. The IPv6 addressing architecture is defined in RFC 4291, which obsoletes RFC 3513.