IPv4 Address Space
IPv4 is based on a 32-bit address scheme that could in theory enable a total of 4 billion hosts (exactly 4,294,967,296) on the whole Internet. However, this 32-bit scheme was originally divided into five hierarchical classes managed by the Internet Assigned Numbers Authority (IANA). The first three classes (A, B, and C) are available as globally unique unicast IP addresses. These classes were assigned to the requesters with a fixed prefix length using different netmask values. A netmask is consecutive series of bits preset to 1 designed to “mask” the network part of an IP address.
Table 1-1 shows the five classes of IPv4 addresses, along with their associated ranges and network masks.
| Classes | Range | Netmask |
|---|---|---|
| A | 0.0.0.0 to 127.255.255.255 | 255.0.0.0 |
| B | 128.0.0.0 to 191.255.255.255 | 255.255.0.0 |
| C | 192.0.0.0 to 223.255.255.255 | 255.255.255.0 |
| D | 224.0.0.0 to 239.255.255.255 | — |
| E | 240.0.0.0 to 255.255.255.255 | — |
| The IANA is an organization dedicated to the central coordination of the Internet. The IANA is responsible for assigning numbers to protocols and blocks of IP addresses to regional Internet registries and large providers. You can find more information about IANA at www.iana.org. | ||
In North America, where early adopters of the Internet were significant in the 1980s, almost all universities and large corporations received Class A or B addresses, even if they had a small number of computers. Today, these same organizations still have unused IPv4 addresses in their assigned blocks of IPv4 addresses, but they have not redistributed them to other organizations. Moreover, many organizations and companies that received an IPv4 address in the 1980s don't exist as such anymore. For example, Digital got bought by Compaq, which might get bought by Hewlett-Packard. Digital and Hewlett-Packard each have a Class A block of addresses.
The redistribution of unused address space is a very important issue of the Internet. In theory, it should be possible to have a global Internet routing table with 4.2 billion entries, but in real life, this represents issues of scalability, performance, and management for large network operators. How is it possible to converge a 4.2 billion-entry database in just a few milliseconds? Just the addition of hundreds of thousands of Class C addresses originating from Class B addresses into the global Internet routing table means doubling the current size of the routing table.
The number of unused IPv4 addresses within these assigned blocks of IPv4 addresses is very large.
Moreover, other large parts of the addressing scheme are not used to assign unique addresses to devices, which decreases the percentage of IPv4 addresses really available as globally unique unicast IP addresses. For example, Class D and E addresses are reserved for multicast and experimental purposes. Networks 0.0.0.0/8, 127.0.0.0/8, and 255.0.0.0/8 are reserved for protocol operations, and 10.0.0.0/8, 169.254.0.0/16, 172.16.0.0/12, 192.168.0.0/16, and 192.0.2.0/24 are special allocations for private networks (defined by RFC 1918). In fact, the sum of all already-assigned Class A and B addresses, unused address spaces, and reserved IP addresses has forced regional Internet registries and ISPs to put a hold on address assignments and distribution. Only small blocks of IPv4 addresses are assigned to organizations, which often means fewer addresses than hosts.
NOTE
Three regional Internet registries in the world are responsible for assigning blocks of IP addresses to providers and organizations. ARIN (American Registry for Internet Numbers) serves North America, Central America, and South America; RIPE NCC (Réseaux IP Européens Network Coordination Center) covers Europe and Africa; and APNIC (Asia Pacific Network Information Center) covers IP address assignment in Asia. All three registries have guidelines for the request of IP address spaces. You can find additional information about these registries at www.arin.net, www.ripe.net, and www.apnic.net.
The 32-bit address space of IPv4, like any other addressing scheme such the telephone numbering system, is not optimal. Christian Huitema proposes a logarithmic ratio that is applied to other address spaces, such as the one used in telephone numbers, to compare the efficiency of use.
Each addressing plan has several levels of hierarchy where some margin is provided. However, over time, the hierarchy might change due to growth and the need for mobility. Then, when an allocation is exceeded, a renumbering is needed that involves a very painful and costly operation. Renumbering of telephone area codes in North America has been done since the 1990s mainly due to the growth of new phone service.
At each level of a hierarchy, there is a loss of efficiency. When several hierarchies are present in an addressing plan, the loss of efficiency is much greater. This has a multiplicative effect on the overall efficiency.
IPv4 is not worse or better than other addressing schemes, but with the class hierarchy (A, B, C, D, and E), in which the most-significant bits of address hierarchy levels are assigned to providers and low-order bits are used for sites and subnets, the address space is less efficient. RFC 3194, The Host-Density Ratio for Address Assignment Efficiency: An update on the H ratio, presents detailed information about the HD ratio and the IPv4 address scheme.
The HD ratio is a percentage used to identify the pain level caused by a specific efficiency. A ratio lower than 80% is manageable, but a ratio higher than 87% is hard to sustain. RFC 3194 states that IPv4's 32-bit address space will reach the maximum pain level when 240 million globally unique unicast IP addresses are used on the Internet.
Current IANA IP Address Space Allocation
Figure 1-1 shows the IANA allocation of IP address space in September 2002. Classes D and E, which are unavailable as globally unique unicast addresses, represent a total of 12% of the whole IPv4 address space. The 2% of unusable addresses includes 0.0.0.0/8, 127.0.0.0/8, 255.0.0.0/8, and private address spaces. The biggest slice of the graph (58%) represents the address space already assigned to organizations and regional Internet registries such ARIN, APNIC, and RIPE, meaning that 28% of the remaining IPv4 space is still unallocated.
Figure 1-1. Percentage of the IPv4 Address Space Assigned in September 2002
Source: Computed from information published by IANA about IPv4 address space allocation
Future Growth of the Internet
The current situation shows that it will be much harder to get addresses in the future as they become a scarce resource and the Internet continues to grow globally. The problem is obvious in some places, but not in North America, where 75% of the IPv4 address space is allocated for less than 10% of the world population.
Moreover, temporary and semipermanent connections such as dialup are being replaced by connections such as cable-modem/xDSL, which require one permanent IP address per node instead of one temporary IP address for a pool of PPP subscribers. The ratio of subscribers per IP address changes from many:1 to 1:1. Wireless networks are emerging markets, and 802.11b devices and mobile networks are deployed everywhere. However, wireless devices frequently change physical locations, access points, and logical subnets during movement, which means that extra pools of IP addresses are requested for these devices.
Some ISPs are running out of IP addresses; therefore, they must assign private addresses to their customers through NAT. New large networks cannot get IPv4 addresses from regional Internet registries or ISPs. New technologies such PDAs, wireless devices, cellular phones, VoIP, and videoconferencing over IP applications, require globally unique unicast IP addresses. Moreover, the current generation of PCs and operating systems allows people to have their own web servers for their personal data. This also requires permanent IP addresses to be assigned on home networks.