What is a disadvantage of not using NAT in a private network?

The selected answer highlights a significant aspect of network design when it comes to security and external communications. Not using Network Address Translation (NAT) in a private network means that the internal IP addresses of devices are exposed directly to the internet if those devices are configured to communicate with external networks. This exposure can lead to vulnerabilities, as malicious actors can directly interact with internal devices, potentially leading to unauthorized access or attacks.

NAT acts as a mediator between the internal network and the external world by translating private IP addresses to a single public IP address. This not only conserves the number of public IP addresses used but also adds a layer of security since the internal structure of the network is hidden from outside parties. In scenarios without NAT, the risk of external threats to the internal network increases significantly since each device's IP is accessible.

While the other options touch on issues like routing complexity, connectivity limitations, and internet access challenges, they do not directly address the security implications of exposing internal IP addresses to the internet, which is a critical consideration in network design. Thus, the correct answer underscores the necessity of NAT in protecting the integrity and confidentiality of a private network's communications.