As reflected in Table 3.1, there are five IP address classes. The high-order bits in the first octet determine this arrangement—thus, any address with the first bits equal to 10 in the first octet belong to Class B. The bit value is significant in determining the major class of the network. Note that the high-order bits in Table 3.1 reflect the binary representation of the number—for example, 00000001 in binary equals 1 in decimal. Without changing the first bit from a 0 to a 1, the highest number that can be represented is 127; however, this is reserved and not part of the Class A space, shown in the first column. The decimal range of the numbers available with the shown high-order bits is presented in the third column.

As a result, the designer should be able to identify that the address 131.192.210.13 is in Class B and that, using the natural mask, the network portion of this address is 131.192.0.0. Notice that the address class is independent of the subnet mask—the mask modifies only the subnet (or supernet) parameters. A supernet is created by inverting the subnet mask to take bits from the natural network portion of the address. Thus, a supernet of 192.168.2.0 and 192.168.3.0 would be presented as 192.168.2.0 255.255.254.0, rather than the natural mask of 255.255.255.0.

IP Network Classes

The IP protocol, version 4, was designed around the concept of network classes in order to provide a natural boundary that all routers could use. This was slightly better than the flatter area-code model used by the telephone company, wherein each area may contain only 10 million numbers and each sub-area is limited to 10 thousand numbers.

	Examples using phone numbers are based on the North American numbering plan. Countries based on other numbering plans typically share the characteristics of this model but may not provide the same number of available addresses.

The early designers of the Internet realized that some sites may need thousands of subnets, or prefix (sub) areas. Others, they reasoned, might need only one or two. This strategy evolved into the five address classes noted in Table 3.1, which have the following characteristics.

Class A Addresses

Class A addresses contain a 0 in the first bit of the first octet. These IP addresses are presented as 0-126 in the first octet. Designers like Class A address blocks because they allow the most flexibility and largest range of addresses, particularly when classful routing protocols are in use. However, assignments in Class A also waste a huge number of addresses— addresses that go unused. This single factor has led to the development of IP v6 and other techniques to extend the life of IP v4, including CIDR (Classless Internet Domain Routing), RFC 1918 addresses, and network address translation (NAT).

	The network address 127.0.0.0 is reserved for the loopback function. This feature is used for diagnostic purposes and typically encompasses the single address of 127.0.0.1. However, any address in the range is reserved for the function.

Class B Addresses

Class B addresses contain a 1 in the first bit of the first octet and a 0 in the second bit of the first octet. These IP addresses are presented as 128-191 in the first octet. The benefit to Class B addresses becomes clear in larger organizations. These addresses provide a broad block of addresses for the organization while attempting to reduce the waste caused by Class A block sizes—few organizations need the volume of addresses provided by Class A blocks.

Class C Addresses

Class C addresses contain a 1 and a 1 in the first two bits of the first octet and a 0 in the third bit of the first octet and range from 192 to 223 in decimal notation. Up to 254 hosts may be assigned within the class, assuming that the entire subnet is equal to the major network. Under the current addressing allocations, Class C address blocks are easier to obtain than Class A or B allocations but are very limited for most organizations. Therefore, companies generally receive a block of contiguous Class C blocks, which are summarized as a supernet. This is also referred to as CIDR.

Class D Addresses

Class D addresses are reserved for IP multicast. Additional information regarding multicast is presented in Chapter 13.

Class E Addresses

Class E is reserved for future use and is currently undefined.

Subnetting in IP

The idea of subnetting in IP is perhaps the concept most misunderstood by new administrators and designers. Unlike AppleTalk and IPX, IP addresses are assigned at both the network and host levels. In AppleTalk and IPX, the administrator or designer need only assign the network-level address. An interesting twist on these protocol characteristics is that the control that IP offers designers can also be a hindrance in that more must be manually configured. This manual process requires decisions and sets limitations that are not present in AppleTalk or IPX.

As will be described in Chapter 6, IPX addresses are a combination of the MAC (Media Access Control) layer address (hardware address) and the IPX network number, which is assigned by the administrator on the router. A virtually unlimited number of hosts may become members of an IPX network.

AppleTalk is slightly more limited in that the administrator or designer assigns a cable range. Each range supports over 250 hosts, as described in Chapter 5. While this assignment requires additional planning, there is generally little need to conserve addresses in AppleTalk as there is with IP. Therefore, no penalty is associated with allocating cable ranges that will support thousands of hosts—the implementation of which is highly unadvised.