This post is part of my note taking while studying for the AWS Certified Advanced Networking - Specialty certification.
Here are some good things to know on AWS subnet addressing.
AWS VPC subnets reserve ‘5’ IPs by default. Why the quotes? It counts the network address and broadcast address:
- First - commonly called ‘.0’ (but isn’t always), is the network address and cannot be used for a host.
- Second - commonly refered to as ‘.1’ (see note above), is used for the VPC router.
- Third - commonly refered to as ‘.2’, is used for Amazon’s DNS servers.
- Fourth - ‘.3’, is reserved for future use.
- Last - ‘.255’ (gotta love those /24’s) is the broadcast address. While broadcast is not supported in VPC subnets, the address is not available for use.
Subnet Size Limits
You can assign a subnet between a /28 and /16, or between 16 and 65,536 IPs, or between 11 and 63,531 usable.
Elastic Load Balancers
The minimum subnet size for a subnet used with ELB is a /27 with 8 free IPs. You need at least two subnets in different availability zones.