Computer networks and their protection

A network is a set of devices that exchange data. By size: local (LAN) – one building or room (a school computer room, home Wi-Fi); regional (MAN, city or province scale) – for example a system of bank branches or city libraries; global (WAN) – countries and continents, whose biggest example is the Internet, a union of thousands of networks. Data travels by wire (twisted pair, optical fibre) or wirelessly (Wi-Fi, mobile networks, satellite). Optical fibre transmits with light, is not affected by outside noise and is very fast; home networks today usually have 100 Mbit/s – 1 Gbit/s. Protocols are the “language” of the network. IP gives each device an address: an IPv4 address is 32 bits (four numbers 0–255, for example 192.168.1.10), 2³² = 4294967296 in total; since these ran short, IPv6 (128 bits) is being introduced. TCP splits data into pieces, confirms that they arrive and resends lost ones (web, mail); UDP does not confirm but is fast (video calls, games). HTTP is the page-transfer protocol, and HTTPS protects it with TLS encryption: the padlock in the browser shows the connection is encrypted but does not guarantee the site is trustworthy. The old Telnet sent data in the open, so encrypted SSH is used instead. Protection measures: a firewall passes or blocks incoming and outgoing packets by rules; for home Wi-Fi use WPA2 or WPA3 encryption and a long password, and for the router change the default administrator password and update its software; do not enter bank details or passwords on open (passwordless) Wi-Fi; a VPN sends the connection through an encrypted “tunnel”, but get it from a trusted service. Entering someone's network without permission or listening to its traffic is against the law. File transfer time = size (bits) : speed (bits/s); 1 byte = 8 bits.