Nat/Napt - Siemens SCALANCE W780 Configuration Manual

Technical basics

3.6 NAT/NAPT

3.6 NAT/NAPT
What is NAT?
With Network Address Translation (NAT), the IP address in a data packet is replaced by
another. NAT is normally used on a gateway between a private LAN and an external network
with globally valid IP addresses. A local IP address of the internal LAN is changed to an
external global IP address by a NAT device at the gateway.
To translate the internal into the global IP address, the NAT device maintains a translation
list. The address assignment is automatic. You configure the address assignment in "Layer 3
> NAT > Basic (Page 266)".
What is NAPT?
In "Network Address Port Translation" (NAPT) or "Port Address Translation" (PAT), several
internal source IP addresses are translated into the same external source IP address. To
identify the individual source nodes, the port of the source device is also stored in the
translation list of the NAT gateway and translated for the external address.
If several local clients send a query to the same external destination IP address over the
NAT gateway, the gateway enters its own external source IP address in the header of these
forwarded frames. Since the forwarded frames have the same global source IP address, the
NAT gateway assigns the frames to the clients using different port number.
Note
NAT/NAPT is possible only on layer 3 of the ISO/OSI reference model. To use the NAT
function, the networks must use the IP protocol.
When using the ISO protocol that operates at layer 2, it is not possible to use NAT.
If a client from the global network wants to use a service in the internal network, the
translation list for the static address assignment needs to be configured. You configure the
translation list for NAPT in "Layer 3 > NAT > NAPT (Page 268)".
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SCALANCE W780/W740 to IEEE 802.11n Web Based Management
Configuration Manual, 03/2015, C79000-G8976-C267-08