CIDR block (Classless Inter-Domain Routing) interpretation

by XDK 4. August 2019 06:09


CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and its associated routing prefix. The notation is constructed from an IP address, a slash ('/') character, and a decimal number. The number is the count of leading 1 bits in the subnet mask. Larger values here indicate smaller networks. CIDR was developed as an alternative to traditional subnetting. The idea is that you can add a specification in the IP address itself as to the number of significant bits that make up the routing or networking portion.

IPv4 CIDR blocks interpretation :

IPv4 : a.b.c.d
Bits : --> 32 bits

CIDR block Notation : a.b.c.d/prefix where prefix is the number of IP address the network can have.

Example :

IPs calculation for the above CIDR block will be [192(8bits).168(8bits).100(8bits).0(8bits)] = 2^32-22 = 2^10 = 1,024
where 32 -> 8 * 4 bits , 22 -> prefix

Total number of IPs for CIDR block will be 1,024 -> 255 -> 255 -> 255 -> 255
where 0 -> 255 = 256 IPs

Total IPs = 256 * 4 = 1,024

By default, AWS uses 5 IPs out of these 256 IPs, So the number of IPs released will be 256 - 5 = 251

IPs used by AWS:

0 -> Network Address
1 -> Amazon VPC Routing
2 -> Domain Name System (DNS)
3 -> Future use by AWS
255 -> Broadcasting

In AWS, a subnet can have 1 primary CIDR block and 4 secondary CIDR blocks


Amazon Web Service

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