Network security startup Altor Networks on Wednesday said it raised $6 million in a second funding round led by Accel Partners and Foundation Capital.

The Redwood City, California based startup is one of a handful of firms targeting what they claim is a security blind spot for many modern data centers -- virtual traffic.

The group which includes Apani, BlueLane, Catbird Networks, Reflex Security, and Altor, has so far attracted about $100 million in funding.

Many of the startups are already in their B and even C rounds since some of them like Brea, California-based Apani and Atlanta-based Reflex have changed direction slightly to get into the virtual security market.

Traditional real-world network security tools are unable to fully protect traffic exchanged among virtual resources, according to Andreas Antonopoulos, senior vice president of Nemertes Research.

"Once you have a certain number of virtualized servers a lot of the traditional security measures stop working effectively," he said. "In fact static security impedes the benefits of virtualization."

Mr. Antonopoulos believes that VCs are honing in on virtual security firms because they see long-term potential for the technology.

"Traditional perimeter security is getting less adequate because of mobility and virtualization so there is a good chance that virtualized security will ultimately supplant traditional security," he said.

Altor, which was founded by a team of veterans from software security firm Check Point, made its market debut last month.