Classful network
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Classful networking is the name given to the first round of changes to the structure of the IP address in IPv4.
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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
- Classful Online IP Subnet Calculator (http://www.subnet-calculator.com/subnet.php)
- Classful networking defined in RFC 791 (http://www.ietf.org/rfc/rfc0791.txt)