This image shows an Oracle Cloud Infrastructure region containing three availability domains (AD), with a virtual cloud network (VCN) spanning those domains. AD 1 contains five subnets, identified as subnets A through E. ADs 2 and 3 are empty in this diagram.

The subnets are arrayed top to bottom, as follows:

Subnet D, which contains a load balancer, described in the surrounding text as item 2.
Subnet E, which is parallel to Subnet D, contains a Bastion server.
Subnet C, which contains the Siebel AI server and two VM instances. This subnet is described in the surrounding text under item 3.
Subnet B, which contains two Siebel app server instances (described in the surrounding text under item 6), a three-node cluster of a Siebel Cloud Gateway (described in the surrounding text under item 4), and a Siebel filesystem.
Subnet A, which contains the primary database and is described in the surrounding text under item 5.

External web clients access the region via an Internet Gateway while internal web clients must go through a VPN, then a dynamic routing gateway, and then into subnet D. Object storage is independent of any AD but within the VCN.

Traffic is routed from subnet D through Subnets C, B, and A. Access to each subnet layer is enforced by a security list, which is independent of any AD but within the VCN.