Year 8 · Autumn 2

Building the Web · Lesson 4 of 5

Search engines: crawling, indexing and ranking

TheoryTopic 16 The web

Year 8 · Building the Web · Lesson 4 of 5

You will learn to

  • Explain the three jobs of a search engine: crawl, index and rank.
  • Explain why a search looks in the engine's index, not the live web.
  • Identify things that help a page rank higher, like links from trusted sites.
  • Explain how clear headings and alt text help people and crawlers find a page.

Year 8 · Building the Web · Lesson 4 of 5

Success criteria

You have got it when you can say:

  • I can put crawl, index and rank in order and explain each one.
  • I can give two reasons why one page ranks above another.
  • I can improve the headings and alt text on my page so it is easier to find.

Year 8 · Building the Web · Lesson 4 of 5

Retrieval starter

Questions from before. Write your answers, then show them.

  1. Last lesson

    The padlock in the address bar tells you…

    A The site is honestB The connection is encrypted with HTTPSC The site has no viruses

    B. The connection is encrypted with HTTPS

  2. Last unit

    A CPU runs at 2 GHz. How long is one tick?

    A 0.5 nsB 2 nsC 0.2 ns

    A. 0.5 ns

  3. Earlier this unit

    In <img src="cat.png" alt="A sleeping cat">, alt is…

    A A tagB An attributeC A CSS property

    B. An attribute

Year 8 · Building the Web · Lesson 4 of 5

Key words

URL
The full web address of something online.
Protocol
A set of agreed rules for communication.
HTML
Markup language for the content of web pages.
CSS
Language that styles how web pages look.

The explanation · step 1 of 1

Search engines

  1. Crawl: programs follow links from page to page.
  2. Index: record which words appear on which pages.
  3. Rank: order matches by relevance and trust (links from other sites, freshness…).
★Remember

When you search, you search the engine's index, not the live web.

Year 8 · Building the Web · Lesson 4 of 5

Quick quiz · 1 of 3

When you search, a search engine looks in…

  1. A Its index, built earlier by crawlers
  2. B Your browser history
  3. C The live web, right then

A. Its index, built earlier by crawlers

Crawl → index happen in advance. Searching just looks words up in the index, then ranks the matches.

Year 8 · Building the Web · Lesson 4 of 5

Quick quiz · 2 of 3

In https://www.bbc.co.uk/news, the path is…

  1. A https://
  2. B www.bbc.co.uk
  3. C /news

C. /news

The path comes after the domain. It says which page on that server.

Year 8 · Building the Web · Lesson 4 of 5

Quick quiz · 3 of 3

404 means…

  1. A Not found
  2. B OK
  3. C The server crashed

A. Not found

The server works, but there's no page at that path. A crash would be 500.

Year 8 · Building the Web · Lesson 4 of 5

Exit ticket

Answer on paper before you go. Then check.

  1. Which controls how a page looks?

    A CSSB HTMLC DNS

    A. CSS CSS = style. HTML = content and structure.

  2. The web and the internet are…

    A The same thingB The internet runs on the webC The web is a service that runs on the internet

    C. The web is a service that runs on the internet The internet is the network. The web is pages over HTTP, one of many services using it.