Subnets are AZ resilient. A subnet can only be in one AZ. One AZ can have multiple subnets
Subnets need to have CIDR range that is within the VPC CIDR Range. No 2 subnets can have overlapping CIDRs
A IPV6 Subnet can have a /64 CIDR
Subnets within one VPC can by default communicate with other Subnets
5 IPs are reserved
1st Address is reserved as network address
N/W+1 address is used by VPC Router
N/W+2 address is used for DNS
N/W+3 address is reserved for future use
Last address in a N/W is reserved as Broadcast address
VPCs have a configuration object associated with them called as DHCP Options Sets. One DHCP Option set is applied to a VPC at one point. This flows to the Subnets within that VPC
DHCP Option sets can be created and associated with VPC. But they can not be edited.