Network Address Translation (NAT)

Correct Answer: Set up a site-to-site VPN using NAT traversal

The best solution for creating a secure communication channel between two networks behind NAT routers is to 'Set up a site-to-site VPN using NAT traversal'. A site-to-site VPN creates an encrypted tunnel between two network sites over the internet, enabling secure communication. NAT traversal allows these sites to connect even when behind NAT routers, which can be a challenge because a NAT changes the IP address. Enabling 'secure peer-to-peer file sharing' would not secure all the network traffic, only the file sharing part, which does not meet the requirement. 'Configure port forwarding for secure communication' is incorrect because port forwarding alone is not a security measure; without encryption provided by a VPN, data transmission can still be intercepted. 'Disable NAT on both routers' would expose both networks to the internet, which raises security risks.

Correct Answer: Insufficient public IP addresses in the NAT pool

The most probable cause for the reported issue is 'Insufficient public IP addresses in the NAT pool'. Network Address Translation (NAT) changes the public IP addresses of the devices in the network to a single public IP address. A dynamic NAT, in particular, uses a pool of public IP addresses and assigns them as needed, and there should be enough public IPs for each internal IP attempting to access the internet. So, if there are insufficient public IP addresses in the NAT pool, the NAT cannot provide an appropriate public IP to an internal device, causing the connectivity issues that the users are experiencing. The other answers are incorrect because: 1) Even if static NAT entries were not removed, they do not interfere with the dynamic NAT operation. 2) UPnP (Universal Plug and Play) technology is not a requirement for dynamic NAT to work properly. It's used to allow devices to automatically discover and connect to each other on a network, which is not related to NAT. 3) Simply disabling and enabling NAT again does not solve the problem if the root cause, such as a deficiency in the pool of public IPs, is not addressed.

Correct Answer: NAT excludes the partner's IP address range

The most likely issue is that Network Address Translation (NAT) excludes the partner's IP address range. NAT is the process of modifying network address information in IPv4 headers while in transit across a traffic routing device, for the purpose of remapping a given address space into another. If the partner's IP address range is excluded from the NAT configurations, users won't be able to access resources in the partner's network. The other answer options are likely incorrect for the following reasons: - 'NAT is not enabled on the router': If NAT was not enabled on the router, then it would generally affect all external resources and not just the resources on the partner's network. - 'Some ports are blocked by the router's firewall': If some ports were blocked by the firewall, it would typically affect specific applications or services, but would not typically prevent all access to resources on an external partner's network unless those resources relied solely on these blocked ports. - 'DNS settings are incorrect': Incorrect DNS settings could certainly prevent users from accessing resources if the resources are accessed via their corresponding Domain Name. However, if the IP address range for the partner's network is known and being used for the access it suggests DNS resolution may not be relied upon, and therefore incorrect DNS settings might not be causing this issue.