Which tunneling protocol is used to carry packets from unroutable IP addresses across a routable IP network?

The correct answer is based on the function and design of GRE (Generic Routing Encapsulation). GRE is a tunneling protocol that encapsulates a wide variety of network layer protocols, allowing data transmission of packets across an IP network. This capability is particularly beneficial for treating unroutable IP addresses, which typically cannot traverse the conventional internet routing structure. By encapsulating these packets, GRE enables communication between networks that may be separated by non-compatible IP networks.

GRE is versatile because it can encapsulate multiple protocols, making it suitable for connecting disparate network architectures. The use of GRE is often seen in virtual private networks (VPNs) and in situations where point-to-point connections are required over a shared infrastructure, allowing for substantial flexibility in network design and implementation.

When considering other protocols listed, while L2TP and PPTP are also tunneling protocols, they primarily focus on the establishment of VPNs and the encapsulation of Layer 2 packets, rather than handling unroutable IP addresses specifically. IPsec, on the other hand, is a security protocol suite that provides confidentiality, authentication, and integrity for IP packets but does not serve the primary role of encapsulating unroutable addresses.

In summary, GRE is the optimal choice for this specific requirement of carrying