When the Heartbleed bug was disclosed on Monday, the attention focused on the fallout for major Internet companies like Yahoo and Amazon. But security experts said the potential for harm could extend much further, to the guts of the Internet and the many devices that connect to it.

By Thursday, some of the companies that make those devices began revealing whether they had been affected.

Cisco Systems, the dominant provider of gear to move traffic through the Internet, said its big routers and servers, as well as its online servers — a big business — were not affected. If they had been, that would have had a significant impact on virtually every major company that connects to the Internet.

Certain products the company makes were affected, it said — some kinds of phones that connect to the Internet, a kind of server that helps people conduct online meetings, and another kind of device used for office communications. Cisco also posted a list of products it had examined for the vulnerability, which it was updating as it continued inspecting its equipment.