Classless Inter-Domain Routing(CIDR), Private, and Public IP

1. Overview

Class Inter-Domain Routing(CIDR) is used for Security Groups rules or AWS networking in general.

They help to define an IP address range

• We've seen WW.XX.YY.ZZ/32 == one IP
• We've seen 0.0.0.0/0 == all IPs
• But we can define for example: 192.168.0.0/26: 192.168.0.0 - 192.168.0.63(64 IP)

2. Component

• The base IP: (XX.XX.XX.XX)
• The Subnet Mask(/26)
• The base IP represents an IP contained in the range
• The subnet masks define how many bits can change in the IP
• The subnet mask can take two forms. For examples:
  • 255.255.255.0: less common
  • /24: more common
• The subnet masks basically allow part of the underlying IP to get additional next values from the base IP
  • /32 allows for 1 IP = 2^0
  • /31 allows for 2 IP = 2^1
  • /30 allows for 4 IP = 2^2
  • /29 allows for 8 IP = 2^3
  • and so on

2.1 Reserved IP

3. Private and Public IP

• The Internet Assigned Numbers Authority (IANA) established certain blocks of IPv4 addresses for the use of private (LAN) and public (Internet) addresses
• Private IP can only show certain values
  • 10.0.0.0 - 10.255.255.255 (10.0.0.0/8) <= in big networks
  • 172.16.0.0 - 172.31.255.255 (172.16.0.0/12) <= default AWS one
  • 192.168.0.0 - 192.168.255.255 (192.168.0.0/16) <= example: home networks
• All the rest of the IP on the internet are public IP

4. Reference

https://www.ipaddressguide.com/cidr

https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing