Alexander, who also serves as the director of the National Security Agency and chief of the Central Security Service, said networks serving the financial community are well-defended, but other sectors are not.
Alexander said the explosion of mobile technology over the past year has nearly doubled the number of opportunities for hackers. He said many people don't realize phones are tied into the same digital network infrastructure as computers.
"The attack surfaces for adversaries to get on the Internet now include all those mobile devices," Alexander said Thursday at the Aspen Security Forum in Colorado. He said landline devices such as desktop computers are better protected than mobile devices.
In assessing the nation's preparedness to handle a serious cyberattack on critical U.S. infrastructure, Alexander said he would give the United States a three on a scale of one to 10.
Alexander said industry and government must work as a team to eliminate risks of cyberattacks because anyone who finds vulnerabilities in the network infrastructure could cause tremendous problems.
Disruptive attacks, like distributed denial-of-service attacks, are aimed at interrupting the flow of communications or finance, but aren't designed to cause long-term damage. Destructive attacks, however, are designed to destroy parts of the network infrastructure, like routers or servers, which would have to be replaced in order to resume normal operations, the Defense Department said in a release. Alexander said replacing the routers and servers could take weeks or months.
Congress is considering bills that would give the Department of Homeland Security a greater role in setting performance requirements for network industries. Alexander said the legislation would make it easier for government and industry to work together to protect from cyberattacks.