Which of the following is a NAT traversal technique commonly used to enable connections across NATs?

• A. UPnP
• B. DMZ
• C. TURN
• D. NAT

Answer: (A)

NAT traversal aims to let devices behind a NAT accept inbound connections from the Internet. Since the NAT blocks unsolicited traffic, a method is needed to create a path through the router's firewall. UPnP, or Universal Plug and Play, lets devices inside the network request the router to open specific ports and create temporary port mappings automatically. This automatic port opening makes it possible for peers or services inside the LAN to be reachable from outside, which is why UPnP is commonly used in gaming and real-time communication to establish connections across NATs. By contrast, DMZ would forward all ports to one device, greatly increasing exposure and risk, and NAT is the device that needs traversal rather than a technique itself. TURN is a fallback relay that forwards traffic through an external server when direct connectivity isn't possible, which adds latency, so it isn't the typical automatic NAT traversal method. So UPnP is the best fit for enabling connections across NATs in a straightforward, automatic way that many consumer routers support.