# Regions, Availability Zones and Subnet Scope — Cloud Networking: VPCs, Subnets & Peering

Source: https://www.skillbyai.com/en/cloud-networking/cn-regions-azs

> Explain regions and Availability Zones and know whether a subnet is zonal or regional on each major cloud.

## Failure domains shape your network

A **region** is a geographic area, such as Mumbai or Hyderabad, containing several **Availability Zones (AZs)**. Each AZ is one or more data centres with independent power, cooling and networking, connected to the other AZs in the region by low-latency links. AZs exist so that a fire or power failure in one building does not take down your whole application. This directly affects subnet design. In AWS a subnet belongs to exactly **one AZ**, so a highly available tier needs one subnet per AZ, for example three app subnets across three AZs. In Azure a subnet spans the whole region and zone choice is made per resource. In Google Cloud a subnet is regional, covering all zones in that region. The practical rule is the same everywhere: spread each tier across at least two zones, and make sure anything that tier depends on (NAT, load balancers, databases) is also zone-redundant.

## Branches of a bank

A bank in Mumbai has several branches in different neighbourhoods. If one branch floods, customers walk to another. A region is the city, AZs are the branches, and putting your servers in only one AZ is like keeping all your cash in a single branch.

## Zonal dependencies hide in plain sight

Teams spread instances over three AZs and then route all of them through a single NAT gateway in one AZ. When that AZ fails, every private instance loses internet access. Check every dependency, not just compute.

**Quiz:** In AWS, what is the scope of a single subnet?

- [x] One Availability Zone
- [ ] The whole region
- [ ] All regions in the account
- [ ] One EC2 instance

*Answer:* One Availability Zone. An AWS subnet lives in exactly one AZ. For high availability you create matching subnets in several AZs.