Add learning reference to CIDR (#5573)

src/data/roadmaps/aws/content/102-vpc/100-cidr-blocks.md

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 # CIDR Blocks
 
-"CIDR" stands for Classless Inter-Domain Routing. In AWS VPC, a CIDR block is the IP address block from which private IPv4 addresses and public IPv4 addresses are allocated when you create a VPC. The CIDR block can range from /28 (16 IP addresses) to /16 (65,536 IP addresses). It represents a network segment and is associated with a network boundary. Upon creation, you cannot change the CIDR block of your VPC, but you can add additional CIDR blocks to it if needed. A VPC's CIDR block should not overlap with any of the existing network's CIDR blocks.
+"CIDR" stands for Classless Inter-Domain Routing. In AWS VPC, a CIDR block is the IP address block from which private IPv4 addresses and public IPv4 addresses are allocated when you create a VPC. The CIDR block can range from /28 (16 IP addresses) to /16 (65,536 IP addresses). It represents a network segment and is associated with a network boundary. Upon creation, you cannot change the CIDR block of your VPC, but you can add additional CIDR blocks to it if needed. A VPC's CIDR block should not overlap with any of the existing network's CIDR blocks.
+
+## References
+
+- [cidr.xyz: Interactive CIDR range visualizer](https://cidr.xyz/)