With Nicira Networks Inc. getting acquired by VMware Inc. (NYSE: VMW), who's left in the software-defined networking (SDN) startup world? (See VMware to Buy SDN Startup for More Than $1B and VMware Insists It's Not Warring With Cisco.)

Given the popularity of SDN as a buzzword, it seems inevitable that a host of small companies will start calling themselves "SDN." Below are a few that have crossed Light Reading's path and seem worth keeping an eye on.

We've allowed the boundaries of "SDN" to stretch a little; you'll find a couple of cloud-networking companies in here that might or might not count, depending on your definitions. Rather than quibble, we'll suggest you use the message boards to tell us who doesn't belong and who we've left out.

Big Switch Networks
Floodlight has become the top OpenFlow controller on the market, in large part because Big Switch Networks seems to be partnered with everybody. Often mentioned in the same breath with Nicira, and likewise staffed with some of the Stanford University students who'd helped create OpenFlow, Big Switch is now the big name among independent SDN companies.

Embrane
Most of the SDN attention has gone to Layer 2, where OpenFlow resides, and Layer 3, which is an obvious next target for OpenFlow or a similar protocol. Embrane Inc. wants to work at Layers 4 through 7, creating virtual appliances on the fly -- things like firewalls or load balancers. Bonus tidbit: Founders including CEO Dante Malagrino come from Cisco Systems Inc. (Nasdaq: CSCO) spin-ins Andiamo and Nuovo and now are (maybe) competing with Cisco's SDN spin-in, Insieme.

Vyatta
Back in 2006, we wrote about Vyatta Inc. putting routing software onto plain servers. Voila -- generic router! Now, Vyatta has shifted to a role helping create network connections between virtual machines. One selling point is that direct east/west connections between virtual machines can be created, as opposed to "tromboning" -- sending the traffic out to a router and back.

Vyatta's age (founded in 2005) makes the "startup" moniker debatable, and the company does describe itself as being about managing virtualization more than about full-blown SDN. "We're not trying to get the network to do programmatic backflips yet," CEO Kelly Herrell says.

ConteXtream
Before the SDN craze kicked in, ConteXtream was talking about applying grid-networking techniques to the cloud. ConteXtream's software, ingeniously named Grid, creates a fabric that connects any network element to any other. The result is a programmable overlay on top of the data-center network.

PLUMgrid
Because PlumGrid Inc. isn't even giving hints about its technology yet, it's easy to pass it off as a bandwagon-hopper. But its team has been contemplating and even developing SDN ideas for about seven years. CEO Awais Nemat was in charge of the switch-chip division at Marvell Technology Group Ltd. (Nasdaq: MRVL), and CTO Pere Monclus and Director of Product Management Valentina Alaria worked on OpenFlow while at Cisco.

Midokura
Probably the least well-known company on this list, Midokura started examining the potential for SDN in 2009. It's based in Japan, where SDN has a strong grip on the industry's collective consciousness and a big-name backer in NTT. Midokura's claim to fame will be the ability to create virtual appliances for Layers 2 through 7 -- overlapping the target markets of Nicira and Embrane, in a sense.

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— Craig Matsumoto, Managing Editor, Light Reading