Internet Infrastructure Basics

When you type an address, your device asks a DNS service for the server's address, opens a connection, sends a request and receives the page in many small packets that are put back together.

• Packets: data is cut into small pieces, each marked with where it came from, where it goes and its order.
• IP gives every device an address. TCP makes sure all pieces arrive. HTTP is the language of the request; HTTPS is HTTP scrambled (encrypted) so others cannot read it.
• Four steps to remember: find the address (DNS), connect (TCP), ask (HTTP request), receive (response).
• A page can fail at any step. Knowing the order lets you test step by step.

Activity. Draw the four steps for a website you use daily and write one failure message for each step.

Standard to meet. Order all four steps correctly and match each failure message to the step where it happens.

DNS is the phone book of the internet. A record tells the world which server address belongs to a name and where email for the name should go.

• A record: name to an IPv4 address (four numbers, such as 192.0.2.10). CNAME: one name is an alias for another name. MX: where email goes, with a priority number.
• TTL (time to live) in seconds says how long others may remember the answer. 3600 is one hour. Lower it before a planned change.
• Change one record at a time and write down the old value first.
• Never delete an MX record unless email has moved elsewhere; otherwise email will bounce back.

Activity. Read the records for a domain you manage and write one plain sentence for each record.

Standard to meet. Explain every record correctly; write a change plan with old value, new value, TTL and a check.

A local network joins devices through a switch and a router. The router connects the local network to the internet and gives each device a private address.

• Router: connects networks and hands out addresses. Switch: joins wired devices. Wi-Fi access point: joins wireless devices.
• A subnet such as 192.168.1.0/24 has 2^8 = 256 addresses; two are reserved, so 254 devices can use it.
• Keep a table: device, owner, address, location, date. Unknown devices on the list are a warning.
• Change the router's default login on day one.

Activity. Build the device table for a real network and find any unknown device.

Standard to meet. Complete the table with dates and give each unknown device a decision.

Hosting is renting a computer that is always on and connected. Providers promise an availability percentage; the percentage turns into downtime you can calculate.

• Availability promise 99.9 percent leaves 0.1 percent of 8,760 hours = 8.76 hours of downtime per year.
• 99 percent allows 87.6 hours; 99.99 percent allows about 53 minutes.
• Ask: what is promised, what refund follows a failure, where are backups, where is the data kept.
• Match the plan to the need: a hobby page and an online shop need different promises.

Activity. Convert three availability promises into hours per year and pick one for a case.

Standard to meet. Convert all three correctly to two decimals and argue the pick with a cost figure.

Most small-network harm comes from weak passwords, old software and unprotected pages. A short checklist stops most of it.

• HTTPS: the padlock means traffic is encrypted. Use it on every page that asks for a login or a payment.
• Firewall: blocks traffic you did not ask for. Keep the router and device firewalls on.
• Use a long unique password for each account, a password manager, and a second check (MFA, such as a code sent to your phone) on email and the domain panel.
• Update the router, devices and website software on a set day each month.

Activity. Run the security checklist on a real set-up and list three fixes with dates.

Standard to meet. Answer every checklist line yes or no, date the fixes, and write down no password.

Bandwidth is how much can flow at once; latency is how long one message takes. A page can be slow for either reason.

• Bandwidth is measured in Mbps (megabits per second). Latency is in milliseconds (ms). Under 50 ms is good for most work.
• Big images are the most common cause of slow pages. Compress before upload.
• Test at the same hour three times and take the middle value.
• Record results in a table so changes can be compared.

Activity. Measure speed three times at two times of day and write what the difference tells you.

Standard to meet. Record six readings in a table and match the conclusion to the numbers.

Three commands find most faults. Ping checks if a device answers. Traceroute shows each hop (stop) on the way. Nslookup shows what DNS answers.

• Ping the router first, then a public server, then the site name. The first failing step shows where the fault is.
• If ping to an address (numbers) works but the name fails, the fault is DNS.
• Traceroute that stops at one hop points to that hop or its owner.
• Write command, result and time for each test. A fault without evidence is a guess.

Activity. Create three faults safely (unplug, wrong DNS, wrong address) and trace each with commands.

Standard to meet. Show the commands, results and conclusion in each trace; fix and retest each fault.

Everything above is only useful if it is written down and the data can be recovered. The rule 3-2-1 means three copies, on two kinds of storage, one away from the site.

• 3-2-1 backup: the live copy, a second copy on another device, a third off-site or in the cloud.
• Test a restore once. A backup never restored is not a backup.
• The one-page file: device table, DNS records, hosting plan, security checklist, backup plan, contacts, date.
• Review the file every six months.

Activity. Build the one-page file for your chosen network and test one restore.

Standard to meet. Include all seven parts, record a time for the restore test, and write the file so that a peer can use it without asking you.

On the slide: L1 Basic · How a page travels

Notes: List the four steps a page takes from address to screen. Notes: draw it as DNS, connect, request, response; no tools needed.

On the slide: L2 Awareness · Names and addresses

Notes: Say what DNS records do, including MX for email and TTL. Notes: look up the records of a practice domain and describe three of them in plain words.

On the slide: L3 Foundation · Check a home or office network

Notes: Map a small network and find its main devices and addresses. Notes: a /24 network holds 254 devices; learners count the devices and check the space.

On the slide: L4 Practitioner · Availability and backups

Notes: Explain availability numbers and the 3-2-1 backup rule. Notes: 99.9 per cent allows about 8.76 hours a year; learners test one restore and record the result.

On the slide: L5 Specialist · Trace a fault

Notes: Trace a fault using ping, name lookup and a test plan. Notes: if ping to a number works but the name fails, the fault is in DNS; learners write the steps they took.

On the slide: L6 Advanced · Run changes safely

Notes: Plan a change by lowering TTL first and keeping a rollback plan. Notes: review a change plan for timing, backups and who is told.

On the slide: L7 Super expert · Design a reliable set-up

Notes: Design the network, DNS, backup and security plan for a small organisation. Notes: review for single points of failure and cost; present a one-page file with a yearly test plan.