Information and communication technology cheat sheet

Networks, protocols and digital information.

Network roles

A client requests a service and a server provides one in a client–server model. A device can perform different roles at different times.

A browser requests a page from a web server.

Addressing and naming

An IP address identifies a network interface in an IP network. DNS maps domain names to records used to locate services.

A domain name and an IP address are not the same identifier.

Protocols

Protocols define agreed message formats and behaviours. Different layers solve different communication tasks.

HTTP describes web exchanges; TCP provides a reliable byte stream.

Security goals

Confidentiality limits disclosure, integrity concerns unauthorised change, and availability concerns usable access.

A system can protect secrecy while still suffering an outage.

Packets and routes

Packet switching divides information into units carried across a network. Packets may experience different delays or loss; higher-level protocols determine how applications respond.

A network path is not necessarily a dedicated circuit reserved for one conversation.

DNS, addresses and URLs

A domain name can be resolved to network addresses through DNS. A URL additionally identifies a scheme and resource; these are related identifiers with different roles.

https://example.org/notes includes a scheme, host and path.

Authentication and authorisation

Authentication establishes an identity or credential claim; authorisation determines what that identity is permitted to do. Successful sign-in does not imply access to every resource.

A user may be allowed to read a shared file but not edit it.

Backup and availability

Backups preserve recoverable copies; redundancy helps a service continue operating. A duplicated system can repeat an accidental deletion unless recovery copies are independently protected.

Define what can be restored and test restoration, rather than assuming that “synced” means “backed up”.

Worked example: ideal transfer time

At 100 Mbit/s, a 250 MB file contains 2000 Mbit using decimal units. The ideal transfer time is 2000/100 = 20 s, before protocol overhead or other limits.

Eight bits make one byte; MB and Mbit are not interchangeable units.

Common mistake: bandwidth guarantees speed

Nominal link capacity is not the same as application throughput. Congestion, latency, server capacity, overhead and packet loss can limit the observed result.

State both the measurement and its conditions when comparing connections.