What are the three major categories of network attacks?

Reconnaissance, access, and denial of service (DoS) attacks.

* Recon maps the target, access manipulates it, and DoS denies it to others. *

1. Reconnaissance attacks - Discovery and mapping of systems, services, or vulnerabilities (using tools like nslookup, whois, and ping)

2. Access attacks - Unauthorized manipulation of data, system access, or user privileges

3. Denial of Service (DoS) - Disabling or corruption of networks, systems, or services

Go deeper:

• Computer security — Wikipedia — covers each of these attack families (information gathering, access/privilege escalation, denial of service) in depth.