请问在网络层能实现NAT吗？

     在网络层怎样实现NAT。
     大家讨论讨论！

网络层，这概念挺模糊的呀。。。


Physical Layer
This is the lowest layer of the OSI model. This layer is involves the reception and transmission of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium. The physical layer carries the signals for all the higher layers. Under Windows 2000 and later versions, the physical layer is implemented by the network interface card (NIC), its transceiver, and the medium to which the NIC is attached. For network components that use serial ports, the physical layer can also include low-level network software that defines how the serial stream of bits is divided into packets of data.
Data Link Layer
This OSI layer is further divided by Institute of Electrical and Electronics Engineers (IEEE) into two sublayers: LLC and MAC. The LLC sublayer provides error-free transfer of data frames from one node to another. The LLC sublayer establishes and terminates logical links, controls frame flow, sequences frames, acknowledges frames, and retransmits unacknowledged frames. The LLC sublayer uses frame acknowledgement and retransmission to provide virtually error-free transmission over the link to the layers above. The MAC sublayer manages access to the network media, checks frame errors, and manages address recognition of received frames. In the network architecture used by Windows 2000 and later versions of the operating system, the Logical Link Control sublayer is implemented in the transport driver, while the Media Access Control sublayer is implemented in the network interface card (NIC). The NIC is controlled by a software device driver called the NIC driver. Windows 2000 and later versions of the operating system ship with NIC drivers for a number of popular NICs.
Network Layer
This layer controls the operation of the subnet. It determines the physical path the data should take, based on the following:
network conditions
priority of service
other factors, including routing, traffic control, frame fragmentation and reassembly, logical-to-physical address mapping, and usage accounting
Transport Layer
This layer ensures that messages are delivered error-free, in sequence, and with no loss or duplication. It relieves the higher-layer protocols from any concern with the transfer of data between them and their peers. A minimal Transport Layer is required in protocol stacks that include a reliable Network or Logical Link Control sublayer that provides virtual circuit capability. For example, because the NetBEUI transport driver for Windows 2000 and later versions includes an OSI-compliant LLC sublayer, its Transport Layer functions are minimal. If the protocol stack does not include an LLC sublayer, and if the Network Layer is unreliable and/or supports datagrams (as with TCP/IP\'s IP layer or NWLink\'s IPX layer), the Transport Layer should include frame sequencing and acknowledgment, as well as retransmission of unacknowledged frames.

谢谢斑竹。
本来我得理解是网络层＝IP层