Worksheets › Class 12 › Computer Networks › Cheat sheet
Class 12 Computer Networks: cheat sheet
CBSE Computer Science. How networks grew, PAN, LAN, MAN and WAN, network devices, bus, star, tree and mesh topologies, MAC and IP addresses, DNS, URLs and the web.
Key ideas
- Networking began with ARPANET in 1969; with NSFNET it grew into the Internet, a network of networks.
- By the area covered: PAN about 10 m around a person, LAN a room to a campus (about 1 km), MAN a city, WAN countries and continents.
- A modem turns digital signals into analog and back; a repeater boosts a weak signal; a hub sends to every computer, a switch only to the one addressed.
- A router forwards data between networks along the best path, and a gateway joins networks that use different protocols.
- Topologies: bus (one backbone cable), star (a central hub or switch), tree (stars joined in a hierarchy), mesh (every node linked to every other).
- A MAC address is 48 bits fixed on the network card; an IPv4 address is 32 bits, four numbers from 0 to 255, given by the network.
- A URL is protocol://domain/path; DNS finds the IP address of a domain name, so the browser can reach the web server.
Remember
| Links for n computers | mesh: n(n − 1)/2 (10 computers → 45); star with a switch: n; ring: n; bus: one backbone |
| Address sizes | MAC 48 bits (12 hex digits); IPv4 32 bits = 4 bytes; IPv6 128 bits |
| Valid IPv4 | 192.168.10.5 is valid; 192.168.300.5 is not, since 300 is above 255 |
| URL parts | https://learn.example.com/notes/ch1.html: protocol https, domain learn.example.com, path /notes/ch1.html |
| Top-level domains | .com commercial; .org organisation; .edu education; .gov government; .net network; .in India |
| Campus layout | server in the block with the most computers; a repeater on any cable over 100 m; a switch in each block |
Watch out
- Using n(n − 1) for mesh links: each link joins two computers, so halve it.
- Mixing up a hub (to every computer) with a switch (only to the one addressed).
- Giving the whole URL, or the path, as the domain name.
- Calling an address valid when one of its parts is above 255.
Worked example
A school has four blocks: Junior (49 computers), Science (12 computers), Sports (25 computers), Library (38 computers). Junior is 71 m from Library, 107 m from Sports and 92 m from Science. (a) Which block should house the server, and why? (b) Suggest a topology and draw the layout in words. (c) Where would a repeater be needed if signals weaken beyond 100 m? (d) Which device in each block connects its computers? (e) What type of network is this?
Working: the server goes where most computers are; repeaters on long cables
Answer: (a) Junior, as it has the most computers (49), so most traffic stays inside it; (b) a star (or tree) topology with Junior at the centre, a cable from it to each other block; (c) on every cable longer than 100 m: Junior–Sports (107 m); (d) a switch; (e) a LAN.
Practise Computer Networks
Revise with this page, then practise: every Computer Networks sheet in the app is new, with an answer key and working.
Coming soon for Android and iPhone.