Managing NFS and NIS, 2nd Edition - Mike Eisler [13]
IPv4 address classes
Each IPv4 address has a network number and a host number. The host number identifies a particular machine on an organization's network. IP addresses are divided into classes that determine which parts of the address make up the network and host numbers, as demonstrated in Table 1-2.
Table 1-2. IPv4 address classes
Address Class and First Octet Value
Network Number Octets
Host Number Octets
Address Form
Number of Networks
Number of Hosts per Network
Maximum Number of Hosts per Class
Class A: 1-126
1
3
N.H.H.H
126
2563 - 2
2,113,928,964
Class B: 128-191
2
2
N.N.H.H
16,384
2562 - 2
1,073,709,056
Class C: 192-223
3
1
N.N.N.H
2,097,152
254
532,676,608
Class D: 224-239
N/A
N/A
M.M.M.M
N/A
N/A
N/A
Class E: 240-255
N/A
N/A
R.R.R.R
N/A
N/A
N/A
Each N represents part of the network number and each H is part of the address's host number. The 8-bit octet has 256 possible values, but 0 and 255 in the last host octet are reserved for forming broadcast addresses.
Network numbers with first octet values of 240-254 are reserved for future use. The network numbers 0, 127, 255, 10, 172.16-172.31, and 192.168.0-192.168.255 are also reserved:
0 is used as a place holder in forming a network number, and in some cases, for IP broadcast addresses.
127 is for a host's loopback interface.
255 is used for IPv4 broadcast addresses.
10, 172.16-172.31, and 192.168.0-192.168.255 are used for private networks that will never be connected to the global Internet.
Note that there are only 126 class A network numbers, but well over two million class C network numbers. When the Internet was founded, it was almost impossible to get a class A network number, and few organizations (aside from entire networks or countries) had enough hosts to justify a class A address. Most companies and universities requested class B or class C addresses. A medium-sized company, with several hundred machines, could request several class C network numbers, putting up to 254 hosts on each network. Now that the Internet is much bigger, the rules for class A, B, and C network number assignment have changed, as explained in Section 1.3.4.
Class D addresses look similar to the other classes in that each address consists of 4 octets with a value no higher than 255 per octet. Unlike classes A, B, and C, a class D address does not have a network number and host number. Class D addresses are multicast addresses, which are used to send messages to more than one recipient host, whereas IP addresses in classes A, B, and C are unicast addresses destined for one recipient. Multicast on the Internet offers plenty of potential for efficient broadcast of information, such as bulk file transfers, audio and video, and stock pricing information, but has achieved limited deployment. There is an ongoing experiment known as the "MBONE" (Multicast backBONE) on the Internet to exploit this technology.
Class E addresses are reserved for future assignment.
Classless IP addressing
In the early 1990s, due to the advent of the World Wide Web, the Internet's growth exploded. In theory, if you sum the maximum number of hosts per classes A, B, and C (refer back to Table 1-2), the Internet can have a potential for over 3.7 billion hosts. In reality, the Internet was running out of address capacity for two reasons.
The first had to do with the inefficiencies built into the class partitioning. About 3.2 billion of the theoretical number of hosts were class A and class B, leaving about 500 million class C addresses. Most organizations did not need class A or class B addresses, and of those that did, a significant fraction of their assigned address space was not needed. Most users could get by with a class C network number, but the typical small