demilitarized zones (DMZ)

A DMZ is simply a place that is under your control but outside of your internal network and it a place to put servers that you want outsiders to reach but you don't want them to get to your internal network. By limiting access, you can also fine tune monitoring.

However, you would want to expose a portion of these internal applications outside your enterprise for your non-employee users like customers and vendors. For example, Oracle E-Business Suite (flagship Oracle ERP application) has some of the modules like iRecruitment, iSupplier, iStore etc.

In Oracle Application how the nodes (FND_NODES) is to exposed:

>To expose to public use the profile “Node Trust Level”

>Set node to Public/Private (Normal -> private, External -> public)

>Set "Responsibility Trust Level" profile to decide whether to expose Application Responsibility to inside or outside firewall.

Internal Applications Tier
The internal applications tier is the server configured for internal users to access Oracle E-Business Suite. It runs the following major application services:

Web and Forms Services
WebLogic Administration service, Node Manager, Oracle HTTP Server, OPMN, WebLogic Managed Servers
Concurrent Manager Services
Reports and Discoverer Services
External Applications Tier
The external applications tier is the server configured for external users for accessing Oracle E-Business Suite. It runs the following application service:

Oracle HTTP Server
WebLogic components like node manager, managed servers etc.

OPMN
Oracle Process Manager and Notification Server (OPMN) is installed and configured on every tier designated to run the web application.  OPMN provides an integrated way to manage all Oracle Application Server components.  OPMN consists of two main pieces:  the Process Manager and the Notification Server. The Process manager (PM) is the centralized process management mechanism in Oracle Application Server and is used to manage the Oracle HTTP Server. The PM starts, restarts, stops, and monitors every process it manages. It also performs death-detection and automatic restart of the processes. Oracle Notification Server (ONS) is the transport mechanism for failure, recovery, startup, and other related notifications between components in Oracle Application Server.

OHS
Oracle HTTP Server (OHS) is installed and configured on every tier that is designated to run the web application . It provides the key infrastructure required for serving the static and dynamic content generated by Oracle E Business Suite products.

Webgate is an out-of-box client which enforces OAM policies on HTTP resources. Typically, it is installed on the webserver like apache and traps all incoming http traffic before it hits core apache. In this fashion, webgate can enforce OAM policies on the http resources residing on the http server.

Oracle id related reference:
Oracle E-Business Suite 11i Configuration in a DMZ (Metalink Note 287176.1)
Oracle E-Business Suite Release 12 Configuration in a DMZ (Metalink Note 380490.1)
Oracle E-Business Suite Release 12.2 Configuration in a DMZ (Doc ID 1375670.1)