Computer Networks and the Internet
This domain tests topologies, the OSI and TCP/IP models, transmission media, addressing and common protocols.
Models and devices
OSI has seven layers: Physical, Data Link, Network, Transport, Session, Presentation, Application. TCP/IP collapses these into four: Network Access, Internet, Transport, Application. A hub works at the physical layer, a switch at the data link layer (MAC addresses), a router at the network layer (IP addresses). Star topology is the most common LAN layout; a single cable break in a star affects only one node.
Protocols
TCP is connection-oriented and reliable (acknowledgements, retransmission). UDP is connectionless and fast, used for streaming and DNS queries. HTTP/HTTPS, FTP, SMTP (send mail), POP3/IMAP (retrieve mail), DNS (name to IP).
Worked example
Find the network address, broadcast address and usable hosts for 192.168.10.45/26. /26 → 32 − 26 = 6 host bits → block size 2^6 = 64; mask 255.255.255.192. Subnets start at 0, 64, 128, 192. 45 lies in 0–63. Network address: 192.168.10.0. Broadcast: 192.168.10.63. Usable hosts: 192.168.10.1 – 192.168.10.62, i.e. 62 hosts.
Traps
Placing IP at the data link layer; saying UDP guarantees delivery; confusing bandwidth (capacity) with latency; forgetting usable hosts = 2^h − 2; mixing up SMTP (sending) with POP3 (retrieving).