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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Addressing

IPv6 Addressing Overview: Addressing Model and Address Types 1 2 3 IPv6 Address and Address Notation and Prefix Representation

Why Were IPv6 Addresses Made So Large?

However, there are drawbacks to having such a huge address space too. Consider that even with a 64-bit address, we'd have a very large address space: 2⁶⁴ is 18,446,744,073,709,551,616 or about 18 million trillion, still probably more addresses than we will ever need. However, by going instead to 128 bits we have made dealing with IP addresses unruly (as we'll see in the next topic) and we have also increased overhead, since every datagram header or other place where IP addresses are referenced must use 16 bytes for each address instead of the 4 that were needed in IPv4, or the 8 that might have been required with a 64-bit address.

So why the “overkill” of going to 128 bits? The main reason is flexibility. Even though we can have a couple zillion addresses if we allocate them one at a time, that makes assignment difficult. We got rid of class-oriented addressing in IPv4 due to the fact that it wasted address space, which is true. The reality, though, is that being able to “waste” address space is a useful luxury.

Having 128 bits allows us to divide the address space and assign various purposes to different bit ranges while still not having to worry about running out of space. In the topic describing the IPv6 global unicast address format we'll see one way that those 128 bits are put to good use; it allows us to create a hierarchy of networks while still saving 64 bits for host IDs, which has its own advantages.

IPv6 Addressing Overview: Addressing Model and Address Types 1 2 3 IPv6 Address and Address Notation and Prefix Representation