Implementing One-Way NAT: When and Why | Cisco Exam 200-125

When to Implement One-Way NAT

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Under which circumstance should a network administrator implement one-way NAT?



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A. B. C. D.


NAT operation is typically transparent to both the internal and external hosts. Typically, the internal host is aware of the true IP address and TCP or UDP port of the external host. Typically, the NAT device may function as the default gateway for the internal host. However, the external host is only aware of the public IP address for the NAT device and the particular port being used to communicate on behalf of a specific internal host.


"Pure NAT", operating on IP alone, may or may not correctly parse protocols that are totally concerned with IP information, such as ICMP, depending on whether the payload is interpreted by a host on the "inside" or "outside" of translation. As soon as the protocol stack is traversed, even with such basic protocols as TCP and UDP, the protocols will break unless NAT takes action beyond the network layer. IP packets have a checksum in each packet header, which provides error detection only for the header. IP datagrams may become fragmented and it is necessary for a NAT to reassemble these fragments to allow correct recalculation of higher-level checksums and correct tracking of which packets belong to which connection. The major transport layer protocols, TCP and UDP, have a checksum that covers all the data they carry, as well as the TCP/UDP header, plus a "pseudo-header" that contains the source and destination IP addresses of the packet carrying the TCP/UDP header. For an originating NAT to pass TCP or UDP successfully, it must recompute the TCP/UDP header checksum based on the translated IP addresses, not the original ones, and put that checksum into the TCP/UDP header of the first packet of the fragmented set of packets. The receiving

NAT must recompute the IP checksum on every packet it passes to the destination host, and also recognize and recompute the TCP/UDP header using the retranslated addresses and pseudo-header. This is not a completely solved problem. One solution is for the receiving NAT to reassemble the entire segment and then recompute a checksum calculated across all packets.

The originating host may perform Maximum transmission unit (MTU) path discovery to determine the packet size that can be transmitted without fragmentation, and then set the don't fragment (DF) bit in the appropriate packet header field. Of course, this is only a one-way solution, because the responding host can send packets of any size, which may be fragmented before reaching the NAT.

The correct answer is B. When traffic that originates outside the network must be routed to internal hosts.

One-way NAT (Network Address Translation) is used to translate the source IP address of packets that originate from a private IP address space to a public IP address when the packets go out to the internet. It is called one-way NAT because it only translates the source IP address and not the destination IP address.

In this scenario, one-way NAT is required when traffic from outside the network needs to access internal hosts. This is because the internal hosts have private IP addresses that cannot be reached from the internet. By using one-way NAT, the source IP address of the packets from the internal hosts is translated to a public IP address before it is sent to the internet. When the response packets come back from the internet, they are sent to the public IP address, which is then translated back to the private IP address of the internal host.

The other options are incorrect:

A. When the network must route UDP traffic: NAT can be used for both UDP and TCP traffic, so this is not a valid reason to implement one-way NAT.

C. When traffic that originates inside the network must be routed to internal hosts: This scenario does not require one-way NAT as the traffic is already originating from inside the network, which means it is already using private IP addresses that can be routed internally.

D. When the network has few public IP addresses and many private IP addresses require outside access: This scenario does require NAT, but it does not require one-way NAT specifically. Two-way NAT can be used to translate both the source and destination IP addresses of packets, allowing internal hosts to access the internet and allowing external hosts to access internal hosts.