Technical data

Chapter 4. Configuring Special Features 99

When an IP packet comes in through an interface (i.e., the Input interface), the router tries to recognize the

packet. The router then examines the Input filters for this interface and, based on the first Input filter that

matches the IP packet, it decides how to handle the packet (forward or discard it).

If NAT translation is enabled for the Input interface, NAT translation is performed.

Forward Phase

At this stage, the router determines to which interface or link the packets will be sent out using its routing

table. It then applies the Forward filters based on the Input interface information. Next the router applies the

Forward filters based on the Output interface information.

Output Phase

If NAT translation is enabled for the Output interface, then NAT translation is performed

The router

examines the Output filters for this interface and, based on the first Output filter that matches the IP packet, it

decides how to handle the packet.

Configuring Filters with Network Address Translation Enabled

General NAT Information

Network Address Translation is an IP address conversion feature that translates a PC’s local (internal) address

into a global (outside/Internet) IP address. NAT is needed when a PC (or several PCs) on a Local Area

Network wants to connect to the Internet or get to a remote network that uses global, registered addresses:

NAT swaps the local IP address with a global IP address: the IP address and port information that the PC uses

are remapped (changed) to the IP address that was assigned to the router and a new port number is assigned.

Note:

The preceding section,

Filters and Interfaces

, describes how NAT “behaves” for each filtering phase.

Filter Actions

For an IP packet to be forwarded successfully, a filter at each implementation point (Input, Forward, and Output)

must

accept the IP packet.

filter at a particular point matches the incoming IP packet, it is assumed that the packet is accepted.

Each IP filter can initiate one of the following three possible actions:

When the packet is accepted at a filter interface (Input, Forward, or Output), the router lets it proceed for

further processing.

Drop

With Drop, the packet is discarded.

Reject