NEW YORK — Cisco said Tuesday that it plans to buy privately-held OpenDNS for $635 million to boost its network security business.

OpenDNS, based in San Francisco, makes online software that protects devices and computers at companies and organizations against viruses and malware. The deal is expected to close in the first quarter of next year.

Cisco, which makes routers and software, said OpenDNS will combine with its security business. “Together, we will help customers protect their extended network wherever the user is and regardless of the device,” said Hilton Romanski, Cisco’s chief technology and strategy officer in a statement.

Shares of Cisco Systems, based in San Jose, slipped 16 cents to $27.38 in morning trading Tuesday.