Connectivity options

Internet Gateway: Public Internet

Internet Gateway provides a path for network traffic between your VCN and the internet.
Allows traffic from subnet’s components to the internet and from the internet to VCN components.
Instances are able to communicate with the internet, inbound, and outbound.
Define a public subnet: Example: 10.0.1.0/24 which ranges 10.0.1.0 to 10.0.1.255
Usually, this is the DMZ for public internet communications (outbound and inbound)
Usually, use it for the Public Load Balancer
You can have only one Internet Gateway per VCN.
Must add a route to the gateway in the VCN routing table to enable traffic flow. Example route: 0.0.0.0/0 to Internet Gateway
Must add a stateful ingress and egress rule to the security list.

Configured for Private subnet which needs to reach the internet (Outbound only)
Define a private subnet: Example: 10.0.0.0/24 which ranges 10.0.0.0 to 10.0.0.255
Attach NAT Gateway: this enables traffic from the internal network to access the internet but it blocks internet traffic from accessing the internal network.
Used for internal components (for databases and other internal components that needs to access the internet to download updates and patches)
Hosts on private networks can initiate outbound connections to the Internet and receive responses. But not receive inbound requests initiated from the internet.
Must add a route to the gateway in the VCN routing table to enable traffic flow. Example route: 0.0.0.0/0 to NAT Gateway
You can have multiple NAT Gateway on a VCN, but a subnet can route traffic to only one NAT Gateway.

Make OCI extend an on-premise network, with easy connectivity in both directions.
Other than the internet (example: On-Premise)
Attach a Dynamic Routing Gateway (DRG Gateway) to VCN. VCN can have one DRG and a DRG can be attached to one VCN.
Usually connects to customer internal data center
Dynamic routing gateway is a virtual router that provides a path for private traffic between your network and destinations other than the internet.
Used in

IPsec VPN, also sometimes referred to as site-to-site VPN because you're connecting sort of to sites.
Fast connect A private dedicated connectivity. Having your own high occupancy vehicle lane if the internet is this massive you set of highways
Remote peering with VNC in another region

After attaching the DRG, you must add a route for the DRG in the VCN’s route table to enable traffic flow to DRG
0.0.0.0/0 to DRG
DRG and VCN have a 1:1 relationship

Service Gateway lets resources in VCN access public OCI services such as Object Storage, but without using the internet or NAT gateway.
Any traffic from VCN that is destined for one of the supported OCI services uses the instances's private IP address for routing, travels over OCI network fabric, and never traverse the internet.
The Oracle public services are available with public IP addresses
Internal networks can reach the Oracle public services privately via a Service Gateway
Usecases: Backup DB systems in VCN
Example: Object Storage and All-Region Services, target Service Gateway

Local Peering Gateway: The process to connect two local VCNs (In the same Region).
So that their resources can communicate over private IP addresses
Must not have overlapping CIDRs
Peering is not transitive
Steps:

In each VCN create a Local Peering Gateway
In each VCN, Add a routing rule with a target to Local Peering Gateway with target IP of the second VCN as shown in the diagram below
In each VCN, Need to define a security rule to allow traffic to/from the other VCN CIDR.
Select the Local Peering Gateway and click to establish a connection.

Connect two OCI remote regions
DRG is a virtual router that provides a path for privileged traffic between your VCN and destinations other than the internet.
After attaching a DRG, you must add a route for the DRG in the VCN’s route table to enable traffic flow.
Is the process of connecting two VCN in different regions so that their resources can communicate using private IP addresses
Requires a remote peering connection (RPC) to be created on the DRGs. RPC job is to act as a connection point for a remotely peered VCN
Used for Disaster Recovery
Must not have overlapping CIDRs
The same setup as Local Peering, but use DRG instead.