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Dynamic versus static address translation
Dynamic address
translation
Static address
translation
Applicability of address
translation
14
Network A
Figure 6:
All connections, leaving from an interface where NAT is enabled are called
connections.
Connections arriving on the NAT enabled interface are called
Outgoing connections typically use dynamic address translation. For example a
connection initiated from IP address X in the private network will be translated into
public IP address Y on an enabled public interface. This mapping will be added
dynamically in an internal table of the device and will exist only for the lifetime of
that connection.
This also implies that, when resetting the device, this dynamic – non-persistent –
table entry will be lost. It is necessary to enable address translation on a public
interface to take profit of this dynamic translation.
Connections initiated from the public network (so called incoming connections) make
use of static address mapping. An incoming initiator packet will need to pass the
static address translation table before being forwarded to the private network. This
information is non-volatile and needs to be configured in advance. Different
configuration flavours exist to configure this static mapping (individual, template,
default) and will be discussed further on in the document.
Address translation is used in case:
Multiple private hosts access a public network through the same gateway
(single public address on the gateway).
The inside address is not routable on the outside network.
The user wants to prevent the inside address(es) from outside attacks.
The user wants to avoid network renumbering when changing service
provider.
The user wants to make servers accessible from the outside network.
NAT enabled
interface
Outbound connections
TM
SpeedTouch
Inbound connections
Inbound/outbound connections
Network B
outbound
inbound
connections.
E-NIT-CTC-20040716-0004 v1.0
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