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Classless Inter Domain Routing
CIDR - Classless Inter Domain Routing - was adopted to help ease the load imposed on internet and large network backbone routers by the increasing size of routing tables. A solution to these problems was found in CIDR.
CIDR permits IP- Address aggregation which in turn reduces the size of routing tables and so addresses the problems listed above.
CIDR comes with it its own simplified form of IP network address notation. Instead of using the network address and subnet mask, CIDR notation uses the network address followed by a slash ("/") and the number of mask bits.
CIDR Table
| Netmask | CIDR | # of IP's | Size |
|---|---|---|---|
| 0.0.0.0 | /0 | 4,294,967,294 | The Internet |
| 128.0.0.0 | /1 | 2,147,483,646 | 128 Class 'A's |
| 192.0.0.0 | /2 | 1,073,741,822 | 64 Class 'A's |
| 224.0.0.0 | /3 | 536,870,910 | 32 Class 'A's |
| 240.0.0.0 | /4 | 268,435,454 | 16 Class 'A's |
| 248.0.0.0 | /5 | 134,217,726 | 8 Class 'A's |
| 252.0.0.0 | /6 | 67,108,862 | 4 Class 'A's |
| 254.0.0.0 | /7 | 33,554,430 | 2 Class 'A's |
| 255.0.0.0 | /8 | 16,777,214 | 1 Class 'A' |
| 255.128.0.0 | /9 | 8,388,606 | 128 Class 'B's |
| 255.192.0.0 | /10 | 4,194,302 | 64 Class 'B's |
| 255.224.0.0 | /11 | 2,097,150 | 32 Class 'B's |
| 255.240.0.0 | /12 | 1,048,574 | 16 Class 'B's |
| 255.248.0.0 | /13 | 524,286 | 8 Class 'B's |
| 255.252.0.0 | /14 | 262,142 | 4 Class 'B's |
| 255.254.0.0 | /15 | 131,070 | 2 Class 'B's |
| 255.255.0.0 | /16 | 65,534 | 1 Class 'B' |
| 255.255.128.0 | /17 | 32,766 | 128 Class 'C's |
| 255.255.192.0 | /18 | 16,382 | 64 Class 'C's |
| 255.255.224.0 | /19 | 8,190 | 32 Class 'C's |
| 255.255.240.0 | /20 | 4,094 | 16 Class 'C's |
| 255.255.248.0 | /21 | 2,046 | 8 Class 'C's |
| 255.255.252.0 | /22 | 1,022 | 4 Class 'C's |
| 255.255.254.0 | /23 | 510 | 2 Class 'C's |
| 255.255.255.0 | /24 | 254 | 1 Class 'C' |
| 255.255.255.128 | /25 | 126 | 128 Hosts |
| 255.255.255.192 | /26 | 62 | 64 Hosts |
| 255.255.255.224 | /27 | 30 | 32 Hosts |
| 255.255.255.240 | /28 | 14 | 16 Hosts |
| 255.255.255.248 | /29 | 6 | 8 Hosts |
| 255.255.255.252 | /30 | 2 | 4 Hosts |
| 255.255.255.254 | /31 | 0 | 2 Hosts |
| 255.255.255.255 | /32 | 1 | 1 Host |
The number of IPs availabe for use is limited to n-2 since the first and the last IP address of each subnet is being reserved for netname and broadcast.
Reserved IP- Addresses and Subnets
Some IP addresses and IP ranges are reserverd for special purposes and are limited to this.
First and last IP of an Subnet
A special type of IP address is the limited broadcast address 255.255.255.255. A broadcast involves delivering a message from one sender to many recipients. Senders direct an IP broadcast to 255.255.255.255 to indicate all other nodes on the local network (LAN) should pick up that message. This broadcast is 'limited' in that it does not reach every node on the Internet, only nodes on the LAN. Technically, IP reserves the entire range of addresses from 255.0.0.0 through 255.255.255.255 for broadcast, and this range should not be considered part of the normal Class E range.
0.0.0.0/8 (0.0.0.0 bis 0.255.255.255)
This is the so-called standard route or default gateway.
10.0.0.0/8 (10.0.0.0 bis 10.255.255.255)
172.16.0.0/12 (172.16.0.0 bis 172.31.255.255)
192.168.0.0/16 (192.168.0.0 bis 192.168.255.255)
The IP standard defines specific address ranges within Class A, Class B, and Class C reserved for use by private networks. Hosts are effectively free to use addresses in the private ranges if they are not connected to the Internet, or if they reside behind firewalls or other gateways that use Network Address Translation. A common misconception is that these addresses are not routable. However, while not routable on the public Internet, they are routable within an organisation or site.
127.0.0.0/8 (127.0.0.0 bis 127.255.255.255)
127.0.0.1 is the loopback address in IP. Loopback is a test mechanism of network adapters. Messages sent to 127.0.0.1 do not get delivered to the network. Instead, the adapter intercepts all loopback messages and returns them to the sending application. IP applications often use this feature to test the behavior of their network interface.
169.254.0.0/16 (169.254.0.0 bis 169.254.255.255)
This is is the link-local address range. The intention is to provide an IP address without a DHCP server being available and without having to configure a network address manually. The network 169.254/16 has been reserved for this purpose. Within this address range, the networks 169.254.0.0/24 and 169.254.255.0/24 have been set aside for future use. If a host on an network cannot obtain a network address via DHCP, an address from 169.254.0.0 to 169.254.255.255 is assigned pseudorandomly.
