Computer Systems, Networks, and Security
What Domain 4 tests
Computer Systems, Networks, and Security connects hardware, operating systems, network protocols, and defensive security. Expect items on the fetch–decode–execute cycle, memory hierarchy, IP addressing and subnetting, TCP/IP layers, common protocols, and the confidentiality–integrity–availability triad.
Core rules
- The CPU fetches an instruction, decodes it, executes it, and writes results; registers are fastest, then cache, RAM, and secondary storage.
- An operating system manages processes, memory, files, devices, and permissions. Virtual memory lets programs use more address space than physical RAM.
- TCP is connection-oriented and reliable; UDP is connectionless and does not guarantee delivery. DNS resolves names to IP addresses; HTTP carries web requests; HTTPS adds TLS encryption.
- IPv4 addresses are 32 bits. A /24 has 256 addresses; a /26 borrows two host bits, leaving 6 host bits and 2^6 = 64 addresses per subnet.
- Security goals are confidentiality, integrity, and availability. Symmetric encryption uses one shared key; asymmetric encryption uses a public/private pair; hashing verifies integrity but is not encryption.
Worked example
A host address is 192.168.1.70 with prefix /26. The subnet size is 2^(32−26) = 64 addresses. Subnets begin at .0, .64, .128, and .192, so this host lies in 192.168.1.64/26. The network address is .64 and the broadcast address is .127. Usable host addresses are .65 through .126, or 62 hosts.
Traps
Do not report 64 usable hosts for a /26; two addresses are reserved. Private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16, not “any 192 address.” A firewall filters traffic but does not remove malware. Hashing cannot be reversed to recover a password, and salting changes identical passwords into different hashes.