DEV Community

Cover image for Networking Series - CIDR Block
Pradheepa P for AWS Community Builders

Posted on • Originally published at

Networking Series - CIDR Block


Before working in serverless technologies, most of the applications I worked on were hosted on-prem. I used to interact with the infrastructure team and the questions they ask were sometimes not understandable to the application developers. The talk about firewall, ip addresses, ssl, ftp, connect direct connections are too much to comprehend initially. With serverless applications development though the application developer need not have to know about these terminologies, it's good to understand what's behind the scenes.

AWS opens the doors for the application developers to know about the networking configuration used by their application. Usually in an on-prem applications the networking services are managed by a different team and the developers are not aware of the configuration. The interaction typically happens where the network team share the snippets of the logs whenever an error occurred connecting to our application. But with more applications being developed on cloud services, developers can at least access the lower environment configurations (dev or staging) which is usually mimicked to production. Let us start the series by getting to know what is a CIDR Block.

CIDR Block (Classless Inter-Domain Routing)

While creating VPC in AWS, the second step is to provide the IP CIDR block i.e the range of IP addresses to be allocated to this VPC. It is of the format (IP address followed by / and a number). Most of us knew the IP address format but what does this number signify in CIDR Block.

CIDR Block Range

  • CIDR is an industry standard.

  • The number is the number of bits in an IP address that must match to be considered as part of the selected CIDR block.

  • Each IP Segment i.e the number between the dots is of eight bits that makes the entire segment consisting of 32 bits.

  • Let us take the ip address and represent them in binary as 0000 1010. 0000 0000. 0000 0000. 0000 0000.

  • Let us take the CIDR block notation as The number here mentions that any ip address that has the first 16 digits matched is part of this CIDR IP block. Since the first 16 bits has to remain unchanged, it leaves room for the rest of the 16 digits to take any value. Hence 216 65536 ip addresses are available in this range.

Additional Resources

Top comments (0)