Classful network

From Academic Kids

(Redirected from Classes of IP addresses)

Classful networking is the name given to the first round of changes to the structure of the IP address in IPv4.

Contents

Before classes

Originally, the 32-bit IPv4 address consisted simply of an 8-bit network number field (which specified the particular network a host was attached to), and a rest field, which gave the address of the host within that network. (This format was picked before the advent of local area networks (LANs), when there were only a few, large, networks such as the ARPANET.)

This resulted in a very low count (256) of network numbers being available, and very early on, as LANs started to appear, it became obvious that that would not be enough.

Classes

As a kluge, the definition of the meaning of IP addresses was changed, to allow three different sizes of network number field (and associated rest fields), as specified in the table below (in bits):

Class Leading bits Network number Rest
Class A     0     7     24
Class B     10     14     16
Class C     110     21     8
Class D (multicast)     1110
Class E (reserved)     1111

which allowed the following population of network numbers (note that the actual numbers are slighlty smaller, as some values - e.g. all zeros - are not allowed):

Class Leading bits Total # of network numbers # of addresses per network
Class A     0     128     16,777,214
Class B     10     16,384     65,534
Class C     110     2,097,152     256

The larger network number fields allowed a larger number of networks, thereby temporarily allowing the continued growth of the Internet.

The IP address netmask, which is so commonly associated with an IP address today, was not required, as the mask length was purely a function of the IP address. Any network device could inspect the first few bits of a 32 bit IP address to see which class it belonged to.

The method of determining whether one address belongs to the same physical network as another IP address worked as it had originally (but see subnet). For each address, the portion of the address which contained the network number was determined, and the contents of the rest part of the address was ignored. If the network numbers matched, the two addresses were on the same network.

The replacement of classes

This first round of changes was not enough to work in the long run, however; an IP address shortage still developed. The principal problem was that most sites were too big for a "class C" network number, and received a "class B" number instead. With the rapid growth of the Internet, the available pool of class B addresses (basically 214, or about 16,000 total) was rapidly being depleted. Classful networking was replaced by Classless Inter-Domain Routing, starting in about 1993, to solve this problem (and others).

Early allocations of IP addresses by IANA were in some cases not made very efficiently, which contributed to the problem. (However, the commonly-held notion that some organizations unfairly or unnecessarily received class A networks is not true; most such allocations date to the period before the introduction of address classes, when the only thing available was what later became known as "class A" network numbers.)

Useful tables

Class ranges

The address ranges used for each class are given in the following table, in the standard dotted decimal notation.

Class Leading bits Start End CIDR equivalent
Class A     0     0.0.0.0 127.255.255.255 /8
Class B     10 128.0.0.0 191.255.255.255 /16
Class C     110 192.0.0.0 223.255.255.255 /24
Class D (multicast)     1110 224.0.0.0 239.255.255.255 NA
Class E (reserved)     1111 240.0.0.0 255.255.255.255 NA

Special ranges

Some addresses are reserved for special uses (RFC 3330).

Addresses CIDR Equivalent Purpose RFC Class Total # of addresses
    0.0.0.0 - 0.255.255.255<tt> 0.0.0.0/8 Zero Addresses RFC 1700 A 16,777,216
<tt>   10.0.0.0 - 10.255.255.255 10.0.0.0/8 Private IP addresses RFC 1918 A 16,777,216
  127.0.0.0 - 127.255.255.255 127.0.0.0/8 Localhost Loopback Address RFC 1700 A 16,777,216
169.254.0.0 - 169.254.255.255 169.254.0.0/16 Microsoft APIPA RFC 3330 B 65,536
 172.16.0.0 - 172.31.255.255 172.16.0.0/12 Private IP addresses RFC 1918 B 1,048,576
  192.0.2.0 - 192.0.2.255 192.0.2.0/24 Documentation and Examples RFC 3330 C 256
192.88.99.0 - 192.88.99.255 192.88.99.0/24 IPv6 to IPv4 relay Anycast RFC 3068 C 256
192.168.0.0 - 192.168.255.255 192.168.0.0/16 Private IP addresses RFC 1918 C 65,536
 198.18.0.0 - 198.19.255.255 198.18.0.0/15 Network Device Benchmark RFC 2544 C 131,072
  224.0.0.0 - 239.255.255.255 224.0.0.0/4 Multicast RFC 3171 D 268,435,456
  240.0.0.0 - 255.255.255.255 240.0.0.0/4 Reserved RFC 1700 E 268,435,456

Bit-wise representation

In the following table, n indicates a binary slot used for network ID. s indicates a binary slot used for host ID. X indicates a binary slot. (without specified purpose)

Class A
  0.  0.  0.  0 = 00000000.00000000.00000000.00000000
127.255.255.255 = 01111111.11111111.11111111.11111111
                  0nnnnnnn.ssssssss.ssssssss.ssssssss

Class B
128.  0.  0.  0 = 10000000.00000000.00000000.00000000
191.255.255.255 = 10111111.11111111.11111111.11111111
                  10nnnnnn.nnnnnnnn.ssssssss.ssssssss

Class C
192.  0.  0.  0 = 11000000.00000000.00000000.00000000
223.255.255.225 = 11011111.11111111.11111111.11111111
                  110nnnnn.nnnnnnnn.nnnnnnnn.ssssssss

Class D
224.  0.  0.  0 = 11100000.00000000.00000000.00000000
239.255.255.255 = 11101111.11111111.11111111.11111111
                  1110XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

Class E
240.  0.  0.  0 = 11110000.00000000.00000000.00000000
255.255.255.255 = 11111111.11111111.11111111.11111111
                  1111XXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX

See also

Classless Inter-Domain Routing

External links

de:Netzklasse it:Classi di indirizzi IP

Navigation

Academic Kids Menu

  • Art and Cultures
    • Art (http://www.academickids.com/encyclopedia/index.php/Art)
    • Architecture (http://www.academickids.com/encyclopedia/index.php/Architecture)
    • Cultures (http://www.academickids.com/encyclopedia/index.php/Cultures)
    • Music (http://www.academickids.com/encyclopedia/index.php/Music)
    • Musical Instruments (http://academickids.com/encyclopedia/index.php/List_of_musical_instruments)
  • Biographies (http://www.academickids.com/encyclopedia/index.php/Biographies)
  • Clipart (http://www.academickids.com/encyclopedia/index.php/Clipart)
  • Geography (http://www.academickids.com/encyclopedia/index.php/Geography)
    • Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
    • Maps (http://www.academickids.com/encyclopedia/index.php/Maps)
    • Flags (http://www.academickids.com/encyclopedia/index.php/Flags)
    • Continents (http://www.academickids.com/encyclopedia/index.php/Continents)
  • History (http://www.academickids.com/encyclopedia/index.php/History)
    • Ancient Civilizations (http://www.academickids.com/encyclopedia/index.php/Ancient_Civilizations)
    • Industrial Revolution (http://www.academickids.com/encyclopedia/index.php/Industrial_Revolution)
    • Middle Ages (http://www.academickids.com/encyclopedia/index.php/Middle_Ages)
    • Prehistory (http://www.academickids.com/encyclopedia/index.php/Prehistory)
    • Renaissance (http://www.academickids.com/encyclopedia/index.php/Renaissance)
    • Timelines (http://www.academickids.com/encyclopedia/index.php/Timelines)
    • United States (http://www.academickids.com/encyclopedia/index.php/United_States)
    • Wars (http://www.academickids.com/encyclopedia/index.php/Wars)
    • World History (http://www.academickids.com/encyclopedia/index.php/History_of_the_world)
  • Human Body (http://www.academickids.com/encyclopedia/index.php/Human_Body)
  • Mathematics (http://www.academickids.com/encyclopedia/index.php/Mathematics)
  • Reference (http://www.academickids.com/encyclopedia/index.php/Reference)
  • Science (http://www.academickids.com/encyclopedia/index.php/Science)
    • Animals (http://www.academickids.com/encyclopedia/index.php/Animals)
    • Aviation (http://www.academickids.com/encyclopedia/index.php/Aviation)
    • Dinosaurs (http://www.academickids.com/encyclopedia/index.php/Dinosaurs)
    • Earth (http://www.academickids.com/encyclopedia/index.php/Earth)
    • Inventions (http://www.academickids.com/encyclopedia/index.php/Inventions)
    • Physical Science (http://www.academickids.com/encyclopedia/index.php/Physical_Science)
    • Plants (http://www.academickids.com/encyclopedia/index.php/Plants)
    • Scientists (http://www.academickids.com/encyclopedia/index.php/Scientists)
  • Social Studies (http://www.academickids.com/encyclopedia/index.php/Social_Studies)
    • Anthropology (http://www.academickids.com/encyclopedia/index.php/Anthropology)
    • Economics (http://www.academickids.com/encyclopedia/index.php/Economics)
    • Government (http://www.academickids.com/encyclopedia/index.php/Government)
    • Religion (http://www.academickids.com/encyclopedia/index.php/Religion)
    • Holidays (http://www.academickids.com/encyclopedia/index.php/Holidays)
  • Space and Astronomy
    • Solar System (http://www.academickids.com/encyclopedia/index.php/Solar_System)
    • Planets (http://www.academickids.com/encyclopedia/index.php/Planets)
  • Sports (http://www.academickids.com/encyclopedia/index.php/Sports)
  • Timelines (http://www.academickids.com/encyclopedia/index.php/Timelines)
  • Weather (http://www.academickids.com/encyclopedia/index.php/Weather)
  • US States (http://www.academickids.com/encyclopedia/index.php/US_States)

Information

  • Home Page (http://academickids.com/encyclopedia/index.php)
  • Contact Us (http://www.academickids.com/encyclopedia/index.php/Contactus)

  • Clip Art (http://classroomclipart.com)
Toolbox
Personal tools