Software-defined networking (SDN) pioneer Martin Casado is leaving VMware to become a partner at big-name venture capital firm Andreessen Horowitz. VMware announced the move today after the markets had closed.

Casado is executive vice president of VMware's Networking and Security Business Unit, but he's best known for being a founder of Nicira, one of the startups that helped kick off the SDN craze. Nicira was acquired by VMware for $1.26 billion in 2012.

Casado will be replaced on April 1 by Rajiv Ramaswami, who was general manager of the networking business at Broadcom.

In a post to VMware's blog, Casado notes that he will remain a strategic advisor to VMware and will continue to "evangelize" NSX, VMware's network virtualization product.

"In retrospect, the networking world is a much different place than it was when I started. And while I played only a very small part in this change, I feel so deeply humbled to have been able to participate at all," Casado writes.