Computer Networks and Internet
Computer Networks and Internet covers how devices are connected, how data travels between them, and what common Internet services and protocols do. Questions range from topology comparison and the OSI/TCP-IP layer model to IP addressing and choosing the right protocol for a task.
Core rules
- LAN covers a room or campus, MAN a city, WAN a country or continent; the Internet is a network of networks.
- Star: central switch, easy fault isolation, single point of failure. Bus: one backbone cable, cheap, collisions. Ring: token passing, one break disables the ring. Mesh: many routes, costly.
- OSI layers, bottom up: Physical, Data Link, Network, Transport, Session, Presentation, Application. TCP/IP collapses these into Network Access, Internet, Transport and Application.
- TCP is connection-oriented and reliable, retransmitting lost segments; UDP is connectionless and fast, used for streaming and DNS queries.
- Ports to remember: HTTP 80, HTTPS 443, FTP 20/21, SMTP 25, POP3 110, DNS 53.
- IPv4 is 32-bit dotted decimal; IPv6 is 128-bit hexadecimal. DNS maps names to IP addresses; a switch forwards within a LAN, a router between networks.
Worked example
How many usable hosts does 192.168.10.0/26 provide?
- /26 leaves 32 − 26 = 6 host bits.
- 2⁶ = 64 addresses per subnet; mask is 255.255.255.192.
- Two addresses are reserved (network .0 and broadcast .63), so 62 hosts are usable.
Traps
- Saying a hub and a switch both forward a frame only to the destination port.
- Confusing HTTP (transfer protocol) with HTML (markup language).
- Counting all 64 addresses as usable instead of 62.
- Swapping SMTP (sending mail) with POP3 or IMAP (retrieving mail).