Introduction to Computer Networks
What this domain tests
The exam checks network types and their scale, topology strengths and weaknesses, the function and layer of each device, core protocol behaviour, and basic security measures.
Types and topologies
- PAN covers a few metres around one person; LAN covers a room, building or campus; MAN covers a city; WAN spans countries and underlies the Internet.
- Bus: one backbone cable, cheap, but a cable break disables the network and collisions rise. Star: all nodes connect to a central hub or switch; easy fault isolation, but the centre is a single point of failure. Ring: unidirectional token passing. Mesh: every node links to many others; highly fault tolerant and costly. Tree combines star groups on a backbone.
Devices
NIC joins a host to the medium; hub repeats signals to all ports (layer 1); switch forwards by MAC address (layer 2); router forwards by IP address between networks (layer 3); gateway converts between dissimilar protocols; repeater regenerates a weak signal; firewall filters traffic.
Protocols
TCP is connection-oriented and reliable, with acknowledgement and retransmission; UDP is connectionless and faster but unreliable. IP handles addressing and routing. HTTP uses port 80, HTTPS port 443, FTP 20/21, SMTP 25 for sending mail, POP3 110 and IMAP 143 for receiving, DNS 53 for resolving names to IP addresses.
Worked example
A school wants one campus network with easy fault isolation. Star topology inside each lab, using a switch, is the answer: a failed cable affects only that workstation. A router then connects the campus LAN to the ISP for Internet access. Reasoning: the switch handles local MAC-level delivery, while the router is needed because traffic must leave the local network.
Traps that cost marks
Calling a hub a switch; placing a router at layer 2; choosing UDP for file transfer that must be complete; reversing SMTP and POP3; naming a topology without justifying it against the stated requirement; treating HTTPS as a different protocol from HTTP rather than HTTP over TLS.