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IPv4 address exhaustion
As the number of Internet-connected devices increases, the address space of the current IPv4 version of the TCP/IP protocol suite is rapidly being consumed by allocations for new devices. This creates an IPv4 address shortage, and could conceivably result in IPv4 address exhaustion.
The IPv4 address space has 32 bits, limiting it to an absolute maximum of 232 (roughly 4.3 billion) possible addresses. For both administrative and technical reasons (the latter in large part being related to routing), IPv4 addresses are allocated in blocks which are restricted to sizes which are powers of 2; this leads to many addresses being unused at any given time. In addition to this, substantial parts of the IP address space are not easily usable because of early technical decisions reserving them for private network use, loopback addresses, multicast, and unspecified future uses, which has resulted in some of these limitations being programmed into devices; working around these limitations will require substantial amounts of re-engineering to increase the amount of available address space. Finally, some of the IPv4 address allocations made early in the development of the Internet (in the 1970s), when only blocks of 224 possible addresses (called a /8 in IPv4 address terminology) were supported, led to some institutions that were involved in the development of the Internet having disproportionally large allocations. MIT, for example, has an entire /8 block allocated to it (224 addresses, about 0.39% of the whole internet address space.)
In the late 1980s only three block sizes (/8, /16 and /24) were supported; since most institutions were too big for /24's, this led to rapid consumption of /16 blocks, and it was feared that the IPv4 address space would run out by the early 2000s. This has been addressed by a number of measures introduced to reduce the demand for new IPv4 addresses, including:
- CIDR, which allowed block sizes of any power of two, which led to more efficient allocation of addresses
- tighter control by Regional Internet Registries on the allocation of addresses to Local Internet Registries
- network renumbering , to reclaim large blocks of address space allocated in the early days of the Internet and now no longer needed
- DHCP, which allowed dynamic allocation of temporary-use addresses
- NAT, which allows a large number of hosts behind the NAT device to be globally represented by a single external IP address on the other side of the NAT device
- IPv6, the next version of the Internet Protocol, with many more addresses.
The top-level unit of allocation of IPv4 addresses is the /8 block, a unit of approximately 16 million addresses. As of April 2004, 160 /8 blocks have been allocated out of a possible absolute maximum of 255.
As of 2004, predictions for the exhaustion of the IP address space range from 2016 for unallocated pool exhaustion to 2023 for complete exhaustion of the address space. Historically, however, forward predictions for the date of address exhaustion have been unreliable; predictions from the late 1980s have not been borne out in practice.
New needs for always-on devices, such as ADSL and cable modems, and the proliferation of mobile wireless devices such as laptop computers, PDAs and mobile phones, are putting pressure back on the consumption of IPv4 space. At the same time, the heavily populated countries of Asia such as India and China are rapidly rising in wealth, and it is clear that in time there are likely to be more than 4 billion always-on Internet-connected personal devices in the form of mobile phones alone.
The planned long-term solution to the IPv4 address shortage is a transition to IPv6, with its 128-bit addresses. However, there are massive obstacles to be overcome for IPv6 deployment, including the need for roll-out of pervasive support for IPv6 throughout the Internet and its connected devices, and remaining architectural problems, such as the lack of proper support for IPv6 multihoming.
If IPv6 is not ready in time, a market in IPv4 addresses may need to be established prior to address exhaustion in an attempt to allocate these addresses as efficiently as possible.
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