**12 host bits = 4096 addresses /20 subnet mask <ref>https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#IPv4_CIDR_blocks</ref>
**Cannot increase or decrease the size of your CIDR block once it's defined (You'd have to create and migrate to a new VPC)
**Recommended to use RFC 1918 ranges <ref>The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets: <pre>
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
</pre>
We will refer to the first block as "24-bit block", the second as "20-bit block", and to the third as "16-bit" block. Note that (in pre-CIDR notation) the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 256 contiguous class C network numbers.</ref>
**https://docs.aws.amazon.com/vpc/latest/userguide/subnet-sizing.html
**Solarwinds has an interactive (not obvious) calculator https://www.solarwinds.com/free-tools/advanced-subnet-calculatorand there's a similar calculator at https://www.site24x7.com/tools/ipv4-subnetcalculator.html
*45. [HOL] Create a Custom VPC
*46. VPC Routing Deep Dive
*56. [HOL] Create VPC Endpoint
<br />{{References}}
