Auto Mode VPC, Custom Mode VPC and Shared VPC in GCP

Auto Mode VPC Network

Use Auto mode VPCs only if you need subnets created automatically (in each region) and do not have concerns related to overlapping address spaces (for e.g. – with VPN tunnel setup).

Since subnets of every auto mode VPC network use the same CIDR block of IP addresses, it is not possible to connect auto mode VPC networks to one another.

Custom Mode VPC Network

Having one subnet automatically created in each region is typically overkill.

You plan to connect VPC networks by using VPC Network Peering or Cloud VPN.

Shared VPC Network

Is not really a network. It is a project – and remember, in GCP, projects come with a default network.

A shared VPC network is a project (containing a VPC  network) which can be shared by other projects.

Say you need to provision a firewall instance in a project and need that instance accessible from other projects. A shared VPC Project (with a network) is one way to accomplish this

Summary

This was meant to be a quick recap of the different types of VPC networks in GCP.   In addition to Custom mode, Auto Mode and Shared VPC Networks, GCP allows VPC Peering – which is a private link between two distinct VPC networks.