IP Addressing Crisis and Solutions
This section discusses some of the restraints involved in using IPv4 addressing. It also discusses some of the various methods and solutions that can be used to help get the most out of the depleted IPv4 address pool, such as CIDR, VLSM, route aggregation, and supernetting.
IP Addressing Crisis
Class A and B addresses make up 75 percent of the IPv4 address space. However, a relative handful of organizations, fewer than 17,000, can be assigned a Class A or B network number. Class C network addresses are far more numerous than Class A and B addresses, although they account for only 12.5 percent of the possible 4 billion, or 232, IP hosts, as illustrated in Figure 2-6.
)
Figure
2-6 IP Address Allocation
Unfortunately, Class C addresses are limited to 254 hosts, which will not meet the needs of larger organizations that cannot acquire a Class A or B address. Even if there were more Class A, B, and C addresses, too many network addresses would cause Internet routers to grind to a halt under the weight of enormous routing tables.
Ultimately, the classful system of IP addressing, even with subnetting, could not scale to effectively handle global demand for Internet connectivity. As early as 1992, the Internet Engineering Task Force (IETF) identified two specific concerns:
Exhaustion of the remaining, unassigned IPv4 network addresses. At the time, the Class B space was on the verge of depletion.
The rapid and substantial increase in the size of the Internet routing tables is because of the Internet's growth. As more Class C addresses came online, the resulting flood of new network information threatened the capability of Internet routers to cope effectively.
In the short term, the IETF decided that a retooled IPv4 would have to hold out long enough for engineers to design and deploy a completely new Internet Protocol. That new protocol, IPv6, solves the address crisis by using a 128-bit address space. After years of planning and development, IPv6 promises to be ready for wide-scale implementation. However, IPv6 continues, for the most part, to wait for that implementation.
One reason that IPv6 has not been rushed into service is that the short-term extensions to IPv4 have been so effective. By eliminating the rules of class, IPv4 now enjoys renewed viability.
Classless Interdomain Routing
Routers use a form of IPv4 addressing called Classless Interdomain Routing (CIDR) that ignores class.
CIDR was introduced in 1993 by RFCs 1517, 1518, 1519, and 1520. It was deployed in 1994. CIDR dramatically improves the scalability and efficiency of IPv4 by providing the following:
Replacement of classful addressing with a more flexible and less wasteful classless scheme
Enhanced route aggregation, also known as supernetting or summarization
Supernetting, which is the combination of contiguous network addresses into a new address defined by the subnet mask
The following sections describe route aggregation, supernetting, and address allocation in more detail.
Route Aggregation and Supernetting
CIDR allows routers to aggregate, or summarize, routing information. It does this by using a bit mask instead of an address class to determine the network portion of an address. This shrinks the size of the routing tables used by the router. In other words, just one address and mask combination can represent the routes to multiple networks.
Without CIDR and route aggregation, a router must maintain many individual entries for the routes within the same network, as opposed to one route for that particular network when using CIDR addressing.
The shaded entries in Table 2-2 identify the 16 bits that, based on the rules of class, represent the network number. Classful routers are forced to handle Class B networks using these 16 bits. Because the first 16 bits of each of these eight network numbers are unique, a classful router sees eight unique networks and must create a routing table entry for each. However, these eight networks do have common bits.
Table 2-2 Route Aggregation and Supernetting
|
Network Number |
First Octet |
Second Octet |
Third Octet |
Fourth Octet |
|
172.24.0.0/16 |
10101100 |
00011000 |
00000000 |
00000000 |
|
172.25.0.0/16 |
10101100 |
00011001 |
00000000 |
00000000 |
|
172.26.0.0/16 |
10101100 |
00011010 |
00000000 |
00000000 |
|
172.27.0.0/16 |
10101100 |
00011011 |
00000000 |
00000000 |
|
172.28.0.0/16 |
10101100 |
00011100 |
00000000 |
00000000 |
|
172.29.0.0/16 |
10101100 |
00011101 |
00000000 |
00000000 |
|
172.30.0.0/16 |
10101100 |
00011110 |
00000000 |
00000000 |
|
172.31.0.0/16 |
10101100 |
00011111 |
00000000 |
00000000 |
Table 2-3 shows that the eight network addresses have the first 13 bits in common. A CIDR-compliant router can summarize routes to these eight networks by using a 13-bit prefix. Only these eight networks share these bits:
10101100 00011
Table 2-3 Dotted-Decimal Notation
|
Network Number |
First Octet |
Second Octet |
Third Octet |
Fourth Octet |
|
172.24.0.0/16 |
10101100 |
00011000 |
00000000 |
00000000 |
|
172.25.0.0/16 |
10101100 |
00011001 |
00000000 |
00000000 |
|
172.26.0.0/16 |
10101100 |
00011010 |
00000000 |
00000000 |
|
172.27.0.0/16 |
10101100 |
00011011 |
00000000 |
00000000 |
|
172.28.0.0/16 |
10101100 |
00011100 |
00000000 |
00000000 |
|
172.29.0.0/16 |
10101100 |
00011101 |
00000000 |
00000000 |
|
172.30.0.0/16 |
10101100 |
00011110 |
00000000 |
00000000 |
|
172.31.0.0/16 |
10101100 |
00011111 |
00000000 |
00000000 |
To represent this prefix in decimal terms, the rest of the address is padded with 0s and then paired with a 13-bit subnet mask:
10101100 00011000 00000000 00000000 = 172.24.0.0 11111111 11111000 00000000 00000000 = 255.248.0.0
Therefore, a single address and mask define a classless prefix that summarizes routes to the eight networks, 172.24.0.0/13.
By using a prefix address to summarize routes, routing table entries can be kept more manageable. The following benefits are a result of the summarized routes:
More efficient routing
Reduced number of CPU cycles when recalculating a routing table or when sorting through the routing table entries to find a match
Reduced router memory requirements
Supernetting is the practice of using a bit mask to group multiple classful networks as a single network address. Supernetting and route aggregation are different names for the same process. However, the term supernetting is most often applied when the aggregated networks are under common administrative control. Supernetting takes bits from the network portion of the network mask, whereas subnetting takes bits from the host portion of the subnet mask. Supernetting and route aggregation are essentially the inverse of subnetting.
Recall that the Class A and Class B address space is almost exhausted, leaving large organizations little choice but to request multiple Class C network addresses from providers. If a company can acquire a block of contiguous Class C network addresses, supernetting can be used so that the addresses appear as a single large network, or supernet.
Supernetting and Address Allocation
Consider Company XYZ, which requires addresses for 400 hosts. Under the classful addressing system, XYZ could apply to a central Internet address authority for a Class B address. If the company got the Class B address and then used it to address one logical group of 400 hosts, tens of thousands of addresses would be wasted. A second option for XYZ would be to request two Class C network numbers, yielding 508, or 2 * 254, host addresses. The drawback of this approach is that XYZ would have to route between its own logical networks. Also, Internet routers would still need to maintain two routing table entries for the XYZ network, rather than just one.
Under a classless addressing system, supernetting allows XYZ to get the address space it needs without wasting addresses or increasing the size of routing tables unnecessarily. Using CIDR, XYZ asks for an address block from its Internet service provider (ISP), not a central authority, such as the Internet Assigned Numbers Authority (IANA). The ISP assesses XYZ's needs and allocates address space from its own large CIDR block of addresses. Providers assume the burden of managing address space in a classless system. With this system, Internet routers keep only one summary route, or supernet route, to the provider network. The provider keeps routes that are more specific to its customer networks. This method drastically reduces the size of Internet routing tables.
In the following example, XYZ receives two contiguous Class C addresses, 207.21.54.0 and 207.21.55.0. If you examine the shaded portions of Table 2-4, you will see that these network addresses have this common 23-bit prefix:
11001111 00010101 0011011
Table 2-4 Supernetting and Address Allocation
|
Network Number |
First Octet |
Second Octet |
Third Octet |
Fourth Octet |
|
207.21.54.0 |
11001111 |
00010101 |
00110110 |
00000000 |
|
207.21.55.0 |
11001111 |
00010101 |
00110111 |
00000000 |
When the sample topology shown in Figure 2-7 is supernetted with a 23-bit mask, 207.21.54.0/23, the address space provides well over 400, or 29, host addresses without the tremendous waste of a Class B address. With the ISP acting as the addressing authority for a CIDR block of addresses, the ISP's customer networks, which include XYZ, can be advertised among Internet routers as a single supernet. The ISP manages a block of 256 Class C network addresses and advertises them to the world using a 16-bit prefix:
207.21.0.0/16
)
Figure
2-7 Addressing with CIDR
When CIDR enabled ISPs to hierarchically distribute and manage blocks of contiguous addresses, IPv4 address space enjoyed the following benefits:
Efficient allocation of addresses
Reduced number of routing table entries