Computer Systems, Networks, and Security
Domain 4 covers how hardware and operating systems execute programs, how networks move data, and how security controls protect assets. Questions connect layers and ask you to reason from a scenario.
Core rules
- The CPU repeats fetch, decode, execute, and write-back. Registers are fastest; cache, RAM, and disk form a hierarchy trading speed for capacity. The OS schedules processes, manages virtual memory, and enforces permissions.
- The TCP/IP model is application, transport, internet, and link. TCP provides ordered, reliable delivery; UDP is connectionless and lower overhead. DNS resolves names to IP addresses; HTTP is an application protocol.
- IPv4 addresses have network and host portions. A /24 has 256 addresses; a /26 borrows two host bits for subnetting, creating four subnets of 64 addresses each.
- Security rests on confidentiality, integrity, and availability. Authentication proves identity; authorization grants permissions. Symmetric encryption uses one key; asymmetric uses a public/private pair; hashing is one-way and is not encryption.
- Common controls include least privilege, patching, firewalls, MFA, and backups.
Worked example
Split 192.168.10.0/24 into four equal subnets. Need two subnet bits because 2²=4, so prefix /26 and mask 255.255.255.192. Block size is 256−192=64. Subnets start at .0, .64, .128, and .192. Each has 64 addresses, but .0 and .63 are network and broadcast in the first, leaving 62 usable hosts. Assigning .64/26 as a host address would be wrong because it is a network address.
Traps
Mixing OSI and TCP/IP layer names; treating encryption as hashing; forgetting network and broadcast addresses; assuming HTTPS encrypts the entire path; and confusing authentication with authorization.