Understanding Private IP Addresses CIDR Prefixes: /8, /12, /16
The Internet Assigned Numbers Authority (IANA) has reserved three blocks of IP address space for private internets: 10.0.0.0-10.255.255.255 (CIDR block: 10.0.0.0/8), 172.16.0.0-172.31.255.255 (CIDR block: 172.16.0.0/12), and 192.168.0.0-192.168.255.255 (CIDR block: 192.168.0.0/16).
What is a CIDR Prefix?
CIDR, or Classless Inter-Domain Routing, is a method for allocating IP addresses that is more flexible than the traditional classful addressing scheme. A CIDR prefix is a way of expressing an IP address range using a combination of the IP address and the number of bits used for the network portion of the address.
Private IP Addresses and CIDR Prefixes
Private IP addresses are used for communication within a private network and are not routed on the public Internet. The three reserved blocks of private IP address space can be further divided into smaller subnets using CIDR prefixes.
CIDR Prefix /8
A CIDR prefix of /8 indicates that the first 8 bits of the IP address are used for the network portion, leaving the remaining 24 bits for host addresses within the network. For the 10.0.0.0/8 block, this results in 16,777,216 possible host addresses.
CIDR Prefix /12
A CIDR prefix of /12 indicates that the first 12 bits of the IP address are used for the network portion, leaving the remaining 20 bits for host addresses within the network. For the 172.16.0.0/12 block, this results in 1,048,576 possible host addresses.
CIDR Prefix /16
A CIDR prefix of /16 indicates that the first 16 bits of the IP address are used for the network portion, leaving the remaining 16 bits for host addresses within the network. For the 192.168.0.0/16 block, this results in 65,536 possible host addresses.
Subnetting Private IP Addresses
Private IP addresses can be further divided into smaller subnets using CIDR prefixes. This is known as subnetting. Subnetting allows for more efficient use of the available IP address space and can also improve network security by isolating different parts of the network.
Example of Subnetting
For example, let's say we have a private network using the 10.0.0.0/8 block and we want to divide it into two subnets. We can do this by using a CIDR prefix of /9 for the first subnet and /10 for the second subnet.
Subnet 1: 10.0.0.0/9 (14,336,000 possible host addresses)
Subnet 2: 10.128.0.0/10 (7,168,000 possible host addresses)
Private IP addresses are used for communication within a private network and are not routed on the public Internet. The three reserved blocks of private IP address space can be further divided into smaller subnets using CIDR prefixes. A CIDR prefix is a way of expressing an IP address range using a combination of the IP address and the number of bits used for the network portion of the address.
References
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Type: Online Resource
Title: Classless Inter-Domain Routing (CIDR)
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Type: Online Resource
Title: IP Addressing and Subnetting for New Users