The Internet revision notes

Structure of the Internet

The Internet is an interconnected system of networks. Data is divided into packets so that it can travel across networks using packet switching. Each packet contains control information, including addressing information, together with the data being transmitted. Packets from the same communication may take different routes and are reassembled at their destination.

Routers, gateways and routing

A router is a device that forwards packets between networks. A gateway is a device or system that provides a connection between different networks and can act as an entry or exit point. Routers are used at points where packets must be directed towards another network. Routing is achieved by examining addressing information in packets and choosing a route through interconnected routers towards the destination. A packet may pass through several routers before reaching its destination.

URLs, domain names and DNS

A uniform resource locator (URL) identifies the location of a resource on the Internet. A fully qualified domain name (FQDN) identifies a host or resource using its complete domain name. A domain name is a human-readable name organised into levels, such as a top-level part and further subdomains. An IP address identifies a device or network interface for communication across networks.

The Domain Name System (DNS) provides a service that links domain names with IP addresses, allowing a domain name to be used instead of remembering an IP address. DNS relies on Domain Name Servers, which store or provide the required name information. Internet registries provide services for managing the allocation and registration of Internet names and addresses; they are needed to help maintain an organised and non-conflicting naming and addressing system.

Internet security

A firewall controls network traffic. Packet filtering examines packet information and permits or blocks packets according to rules. A proxy server acts as an intermediary between a client and another network or service. Stateful inspection considers the state of network connections when deciding whether traffic should be allowed.

Symmetric encryption uses the same key to encrypt and decrypt data, so the key must be exchanged securely. Asymmetric encryption uses a related public key and private key. A public key can be made available while the private key is kept secret; key exchange mechanisms allow communicating parties to establish the material needed for protected communication.

A digital certificate associates an identity with a public key and is obtained through a certificate process. It can be used to support trust in the claimed identity. A digital signature is created using a private key and used to show that data came from the claimed sender and has not been altered.

Worms, trojans and viruses are malicious software that exploit vulnerabilities. Improved code quality can reduce weaknesses, while monitoring can help identify suspicious activity. Protection mechanisms can help prevent, limit or remove the effects of these threats.

Common errors

Do not treat a URL, domain name and IP address as interchangeable. Do not say that a firewall is only a packet filter: proxy servers and stateful inspection are also firewall approaches. Do not confuse a public key with a private key, and do not claim that a digital certificate and a digital signature perform the same function.