IP SAN (Internet Protocol Storage Area Network) typically uses Ethernet networks to connect servers and storage systems, utilizing the IP protocol suite. While Fibre Channel SANs often use optical networks, IP SANs do not inherently require such infrastructure. Therefore, the statement that IP SAN uses underlying optical networks is incorrect.

Software Defined Networking (SDN) is best described as the decoupling of the data plane and control plane in networks. This architectural approach allows for more flexible and programmable network management, where the control plane is abstracted from the physical hardware and can manage the network through software, enabling dynamic and efficient control over network traffic and resources.