A-Level Computer Science: DNS, Protocols, Network Security and the Web

This material trains the networking and web technology content of A level Computer Science beyond IP routing: how a name becomes an address, which protocol and port do which job, how networks are protected, and how the web itself works, from a page's code to the way search engines rank it. It is for Year 12 and Year 13 students.

The quiz has twelve questions. The first four cover the internet's structure: what happens in a DNS lookup for a name the browser has never seen, the port used by HTTPS, which TCP/IP layer adds port numbers and resends lost data, and why a peer-to-peer network suits a game company pushing an update to millions of players. The fifth separates the jobs of a firewall and a proxy server, and the last question asks which email protocol keeps messages in step across several devices. Three questions are about search engines: one full PageRank calculation with the formula and link structure given in the question, why a link from a page with many outgoing links is worth less, and how crawlers and an index let queries be answered quickly. Two use code printed in the question: a short JavaScript loop where joining a number to a string gives 123 rather than 6, and choosing the CSS selector that picks out one heading by its id. A final web question explains why data checked in the browser must be checked again on the server.

Every explanation gives the reasoning in full: the PageRank question shows each division by the number of outgoing links, and the JavaScript question traces the string as it grows. The script was run to check its output.

The flashcards cover DNS, protocols, the TCP/IP layers, well-known ports, client-server and peer-to-peer networks, firewalls, proxy servers, the roles of HTML, CSS and JavaScript, CSS selectors, indexing, the PageRank formula and damping factor, and client-side and server-side processing.

The written work has eight longer tasks to answer on paper: a full DNS lookup, the four TCP/IP layers during a page request, client-server compared with peer-to-peer for two situations, firewalls, proxies and encryption, two iterations of PageRank on three pages, how search engines index and rank pages, a small HTML form with JavaScript validation, and choosing tasks for the client and the server. Each task has a model answer and the points a marker would look for.

There is also a short oral practice with an examiner, who gives all code and link structures in full, asks one question at a time and gives brief feedback at the end.

The content is based on the networking and web sections of the A level specifications, for example OCR H446 sections 1.3.3 and 1.3.4 (DNS, protocol layering, network security, client-server and peer to peer, HTML, CSS and JavaScript, search engine indexing, PageRank, server and client side processing) and AQA 7517 section 4.9 (communication and networking).

• Describe how DNS resolves a domain name to an IP address
• Match common protocols to their ports and to the layers of the TCP/IP stack
• Compare client-server and peer-to-peer networks and explain firewalls and proxies
• Explain search engine indexing and calculate PageRank values
• Read short HTML, CSS and JavaScript and predict what a page shows
• Choose between client-side and server-side processing for a task

Practice material written by Zestly, based on the networking and web technology content of the A level Computer Science specifications (for example OCR H446 sections 1.3.3 and 1.3.4, and AQA 7517 section 4.9).