D-Link DFL-1660 User Manual page 371

7.2. NAT
However, since there is a possible range of 64,500 source ports and the same number for destination
ports, it is theoretically possible to have over 4 billion connections between two IP addresses if all
ports are used.
Using NAT Pools Can Increase the Connections
To increase the number of NAT connections that can exist between the NetDefend Firewall and a
particular external host IP, the NetDefendOS NAT pools feature can be used which can
automatically make use of additional IP addresses on the firewall.
This is useful in situations where a remote server requires that all connections are to a single port
number. In such cases, the 64,500 limit for unique IP address pairs will apply.
See Section 7.3, "NAT Pools" for more information about this topic.
The Source IP Address Used for Translation
There are three options for how NetDefendOS determines the source IP address that will be used for
NAT:
• Use the IP Address of the Interface
When a new connection is established, the routing table is consulted to resolve the outbound
interface for the connection. The IP address of that resolved interface is then used as the new
source IP address when NetDefendOS performs the address translation. This is the default way
that the IP address is determined.
• Specify a Specific IP Address
A specific IP address can be specified as the new source IP address. The specified IP address
needs to have a matching ARP Publish entry configured for the outbound interface. Otherwise,
the return traffic will not be received by the NetDefend Firewall. This technique might be used
when the source IP is to differ based on the source of the traffic. For example, an ISP that is
using NAT, might use different IP addresses for different customers.
• Use an IP Address from a NAT Pool
A NAT Pool, which is a set of IP addresses defined by the administrator, can be used. The next
available address from the pool can be used as the IP address used for NAT. There can be one or
many NAT pools and a single pool can be used in more than one NAT rule. This topic is
discussed further in Section 7.3, "NAT Pools".
Applying NAT Translation
The following illustrates how NAT is applied in practice on a new connection:
1. The sender at IP address 192.168.1.5 sends a packet from a dynamically assigned port, for
example 1038, to a server, for example 195.55.66.77 port 80.
2. In this example, the Use Interface Address option is used, and we will use 195.11.22.33 as the
interface address. In addition, the source port is changed to a random free port on the
NetDefend Firewall and which is above port 1024. In this example, we will assume port 32,789
is chosen. The packet is then sent to its destination.
192.168.1.5:1038 => 195.55.66.77:80
195.11.22.33:32789 => 195.55.66.77:80
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Chapter 7. Address Translation