Computer Networks and Data Communications
Core concepts
Know the OSI seven layers and the TCP/IP model, and map protocols to layers: application (HTTP, FTP, SMTP, DNS), transport (TCP, UDP), internet (IP, ICMP), and link (Ethernet, ARP). Understand addressing: MAC addresses at layer 2, IP addresses and subnetting at layer 3, ports at layer 4. Be able to explain TCP's connection-oriented, reliable, ordered delivery versus UDP's connectionless, best-effort service. Study switching versus routing, transmission media, multiplexing, error detection (parity, checksum, CRC), and basic network devices (hub, switch, router, gateway, repeater). Know topologies, LAN/WAN, and the role of DNS and DHCP.
Traps
Examiners test which layer a device operates at - a switch is layer 2, a router layer 3, a hub layer 1. Watch for confusion between IPv4 address classes and private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16). Remember TCP uses a three-way handshake (SYN, SYN-ACK, ACK) and that UDP is preferred for streaming because it avoids retransmission delay. Do not claim DNS resolves MAC addresses - that is ARP.
Worked example
Subnet 192.168.1.0/24 into four equal subnets. Borrow 2 bits, giving /26 and a block size of 64. The subnets are 192.168.1.0, .64, .128, and .192, each with 62 usable hosts (64 minus network and broadcast). Rule of thumb: usable hosts = 2^h - 2, where h is the number of host bits.