Absolutely, let's break down the more complex aspects of Network Address Translation (NAT) at the transport layer, including techniques like port preservation and port forwarding:
Port Preservation:
- Port preservation is used in one-to-many NAT scenarios where multiple internal devices share a single external IP.
- Port preservation is a technique where the source port chosen by a client ****is the same port used by the router.
- It ensures that return traffic from external servers is correctly directed to the appropriate internal device.
- When an internal device initiates an outbound connection, its chosen source port (ephemeral port) is retained by the NAT router.
- The router maintains a table linking internal IP, internal port, external IP, and external port for proper routing.
- Incoming response traffic from external servers uses the external port to determine which internal device to forward the response to.
Port Forwarding:
- Port forwarding is a technique that allows specific incoming ports to be mapped to particular internal devices.
- It enables external users to access services hosted on internal devices while still benefiting from IP masquerading.
- Example: Web server on 10.1.1.5 and router's external IP is 192.168.1.1.
- Port forwarding on the router configures traffic on port 80 (HTTP) to be forwarded to 10.1.1.5.
- External users only need to know the router's IP (192.168.1.1) to access the web server.
- Incoming requests to 192.168.1.1:80 are automatically directed to 10.1.1.5.
- Response traffic has its source IP rewritten to the router's external IP.
Port Forwarding for Different Services:
- Port forwarding allows services on different internal devices to be accessed using a single external IP.
- External traffic aimed at the same external IP can be directed to different internal devices based on destination port.
- For instance, a company with a web server (10.1.1.5) and a mail server (10.1.1.6) can use port forwarding.
- Requests to external IP's port 80 can be directed to the web server, while requests to external IP's port 25 can be directed to the mail server.
- This way, the organization can offer multiple services without exposing internal IPs.