[AWS Subnet] CIDR (Classless Inter-Domain Routing)

In private IP address, used to divide IP address with Classes

 

Class Method

 

Class	Reserved IP address range	Subnet Mask
class A	10.0.0.0 - 10.255.255.255	255.0.0.0
class B	172.16.0.0 - 172.31.255.255	255.240.0.0
class C	192.168.0.0 - 192.168.255.255	255.255.0.0

Classless Method

→ CIDR

 

 

 

CIDR is an IP address allocation method that improves data routing efficiency on the internet.

• CIDR Block: range of IP addresses
• CIDR Notation: range of CIDR Block

 

 

 

[AWS Subnet]

In AWS subnet, the allowed CIDR block sizes for a subnet range from /16 to /28.

• /16: Allows for 65,536 IP addresses (65,534 usable).
• /28: Allows for 16 IP addresses (14 usable).

This means that the smallest subnet you can create has a /28 prefix (which provides 16 IP addresses, 14 of which are usable), and the largest subnet you can create has a /16 prefix (which provides 65,536 IP addresses, 65,534 of which are usable).

https://docs.aws.amazon.com/vpc/latest/userguide/subnet-sizing.html#subnet-sizing-ipv4

Subnet CIDR blocks - Amazon Virtual Private Cloud