Network hardware and the internet

Computer Science AS · Communication. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A LAN needs hardware: a network interface card (NIC) in each device, a switch to pass data to the right device, wireless access points for WiFi devices, and a router to join the LAN to other networks such as the internet. Ethernet is the usual set of rules for a wired LAN, and CSMA/CD is its way of dealing with collisions.

The internet is the worldwide collection of connected networks. The World Wide Web is only one service that runs on it: web pages reached with URLs.

Every device on the internet needs an IP address. People use domain names instead, and the Domain Name Service (DNS) finds the IP address that matches a domain name.

Rules to remember

Switch: sends data only to the device it is addressed to, inside one LAN. Router: forwards packets between networks using IP addresses
Repeater: regenerates a weak signal. Bridge: joins two LAN segments. WAP: lets wireless devices join a wired LAN
CSMA/CD: listen before sending; if a collision is detected, stop, send a jam signal, wait a random time, then send again
Bit streaming: real-time is a live event sent as it happens; on-demand is a stored file the user starts when they choose. The broadband speed must be at least the bit rate, or the video keeps stopping to buffer
IPv4: 32 bits, four denary numbers from 0 to 255 separated by dots. IPv6: 128 bits, eight groups of four hexadecimal digits separated by colons
Public IP address: unique on the internet. Private: used only inside a LAN, not routed over the internet. Static: never changes. Dynamic: may change each time the device connects
URL = protocol + domain name + path and file name; the DNS returns the IP address for the domain name

Common mistake

Students use "internet" and "World Wide Web" as if they mean the same thing. The internet is the hardware and connections; the WWW is the collection of web pages that is reached over it.

Worked example

A laptop on a home LAN has the IP address 192.168.1.20. The home router has the address 82.10.4.7 from the internet service provider. Which address does a web server on the internet see?

192.168.1.20 is a private address, so it is used only inside the LAN and is not routed over the internet.
All traffic leaving the LAN goes through the router.
The router sends the request using its own public address, so replies come back to the router, which passes them to the laptop.

Answer: The web server sees the router's public address, 82.10.4.7.

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