TCP/IP
Chapter 3. Internetworking protocols
3.1 Internet Protocol (IP)
3.1.5 The IP address exhaustion problem
The number of networks on the Internet has been approximately doubling annually for a number of years. However, the usage of the Class A, B, and C networks differs greatly. Nearly all of the new networks assigned in the late 1980s were Class B, and in 1990 it became apparent that if this trend continued, the last Class B network number would be assigned during 1994.
On the other hand, Class C networks were hardly being used.
The reason for this trend was that most potential users found a Class B network to be large enough for their anticipated needs, since it
accommodates up to 65534 hosts, whereas a class C network, with a maximum of 254 hosts, severely restricts the potential growth of even a small initial network. Furthermore, most of the class B networks being assigned were small ones. There are relatively few networks that would need as many as 65,534 host addresses, but very few for which 254 hosts would be an adequate limit. In summary, although the Class A, Class B, and Class C divisions of the IP address are logical and easy-to-use (because they occur on byte boundaries), with hindsight, they are not the most practical because Class C networks are too small to be useful for most organizations, while Class B networks are too large to be densely populated by any but the largest organizations.
In May 1996, all Class A addresses were either allocated or assigned, as well as 61.95 percent of Class B and 36.44 percent of Class C IP network
addresses. The terms assigned and allocated in this context have the following meanings:
• Assigned: The number of network numbers in use. The Class C figures are somewhat inaccurate, because the figures do not include many class C networks in Europe, which were allocated to RIPE and subsequently assigned but which are still recorded as allocated.
• Allocated: This includes all of the assigned networks and additionally, those networks that have either been reserved by IANA (for example, the 63 class A networks are all reserved by IANA) or have been allocated to
regional registries by IANA and will subsequently be assigned by those registries.
Another way to look at these numbers is to examine the proportion of the address space that has been used. The figures in the table do not show, for example, that the Class A address space is as big as the rest combined, or that a single Class A network can theoretically have as many hosts as 66,000 Class C networks.
Since 1990, the number of assigned Class B networks has been increasing at a much lower rate than the total number of assigned networks and the anticipated exhaustion of the Class B network numbers has not yet occurred.
The reason for this is that the policies on network number allocation were changed in late 1990 to preserve the existing address space, in particular to avert the exhaustion of the Class B address space. The new policies can be summarized as follows.
• The upper half of the Class A address space (network numbers 64 to 127) is reserved indefinitely to allow for the possibility of using it for transition to a new numbering scheme.
• Class B networks are only assigned to organizations that can clearly demonstrate a need for them. The same is, of course, true for Class A networks. The requirements for Class B networks are that the requesting organization:
- Has a subnetting plan that documents more than 32 subnets within its organizational network
- Has more than 4096 hosts
Any requirements for a Class A network would be handled on an individual case basis.
• Organizations that do not fulfill the requirements for a Class B network are assigned a consecutively numbered block of Class C network numbers.
• The lower half of the Class C address space (network numbers 192.0.0 through 207.255.255) is divided into eight blocks, which are allocated to regional authorities as follows:
192.0.0 - 193.255.255 Multi-regional 194.0.0 - 195.255.255 Europe 196.0.0 - 197.255.255 Others
198.0.0 - 199.255.255 North America
200.0.0 - 201.255.255 Central and South America
Chapter 3. Internetworking protocols 85 202.0.0 - 203.255.255 Pacific Rim
204.0.0 - 205.255.255 Others 206.0.0 - 207.255.255 Others
The ranges defined as Others are to be where flexibility outside the constraints of regional boundaries is required. The range defined as multi-regional includes the Class C networks that were assigned before this new scheme was adopted. The 192 networks were assigned by the InterNIC and the 193 networks were previously allocated to RIPE in Europe.
The upper half of the Class C address space (208.0.0 to 223.255.255) remains unassigned and unallocated.
• Where an organization has a range of class C network numbers, the range provided is assigned as a bit-wise contiguous range of network numbers, and the number of networks in the range is a power of 2. That is, all IP addresses in the range have a common prefix, and every address with that prefix is within the range. For example, a European organization requiring 1500 IP addresses would be assigned eight Class C network numbers (2048 IP addresses) from the number space reserved for European networks (194.0.0 through 195.255.255) and the first of these network numbers would be divisible by eight. A range of addresses satisfying these rules would be 194.32.136 through 194.32.143, in which case the range would consist of all of the IP addresses with the 21-bit prefix 194.32.136, or B '110000100010000010001'.
The maximum number of network numbers assigned contiguously is 64, corresponding to a prefix of 18 bits. An organization requiring more than 4096 addresses but less than 16,384 addresses can request either a Class B or a range of Class C addresses. In general, the number of Class C networks assigned is the minimum required to provide the necessary number of IP addresses for the organization on the basis of a two-year outlook. However, in some cases, an organization can request multiple networks to be treated separately. For example, an organization with 600 hosts would normally be assigned four class C networks. However, if those hosts were distributed across 10 token-ring LANs with between 50 and 70 hosts per LAN, such an allocation would cause serious problems, since the organization would have to find 10 subnets within a 10-bit local address range. This would mean at least some of the LANs having a subnet mask of 255.255.255.192 which allows only 62 hosts per LAN. The intent of the rules is not to force the organization into complex subnetting of small networks, and the organization should request 10 different Class C numbers, one for each LAN.
The current rules are to be found in RFC 2050 – Internet Registry IP
Allocation Guidelines, which updates RFC 1466. The reasons for the rules for the allocation of Class C network numbers will become apparent in the following sections. The use of Class C network numbers in this way has averted the exhaustion of the Class B address space, but it is not a permanent solution to the overall address space constraints that are fundamental to IP. A long-term solution is discussed in Chapter 17, “IP Version 6” on page 559.