Networks, protocols and security

Network characteristics, LANs and WANs

Networks support shared files, printers, collaboration and centrally managed services. Benefits depend on availability, bandwidth, latency, maintenance and security. Bandwidth is a capacity/rate; latency is delay before data arrives. A high-capacity distant link can still have inconvenient latency for interactive requests.

A local area network (LAN) covers a limited site and is commonly managed by one organisation. A wide area network (WAN) connects geographically separated networks, often through provider infrastructure. The internet is an interconnection of many networks, not one central machine.

A router connecting a school's LAN to its provider forwards traffic beyond the local subnet; internal traffic need not travel through the public internet.

Use the four-layer TCP/IP model

We use four layers: application, transport, internet and link/network access. Other teaching models split the link/physical responsibilities into more layers; keep the named model consistent. Application protocols express the service, such as HTTP for web messages and DNS for name queries.

Transport identifies communicating application endpoints using ports and supplies transport behaviour.

TCP numbers bytes, acknowledges receipt, detects/retransmits loss and presents an ordered reliable stream. IP supplies addressing/routing between networks, without guaranteeing reliable ordered delivery itself. Link protocols frame data for a local link.

Encapsulation adds relevant headers as data moves down the stack, then the receiving side processes them in reverse. A router generally forwards using IP addressing; a LAN switch generally forwards using link-layer MAC addressing.

DNS and routing are separate tasks

A resolver can answer from an unexpired cached record. On a cache miss, it can query the DNS hierarchy. Caching reduces repeated work, but changed records may not reach every user immediately.

After finding an address, the application can contact the destination. DNS has not sent the requested web page and has not computed a packet route. Routers use forwarding/routing information to choose next hops.

A destination IP identifies a network endpoint; a destination port distinguishes a service/process endpoint on that host. The domain name, IP address and MAC address perform different jobs.

Packet and circuit switching

Packet switching divides data into addressed packets that share network capacity. Routers forward each packet; congestion can create queueing/loss and packets may follow different routes. Higher-layer mechanisms can reconstruct the intended data. Shared capacity suits bursty traffic, but delays can vary and headers impose overhead.

Circuit switching establishes a path with reserved resources for the communication session. This can offer predictable capacity once established, but setup takes time and reserved capacity can remain unused during silence.

Compare resource reservation and usage patterns rather than saying packet switching is always faster or circuit switching is immune to failure.

Hardware has distinct roles

A network interface controller (NIC) connects a device to a network and implements link-level functions. A switch connects devices within a LAN and learns MAC-to-port mappings to forward frames. A router connects networks and forwards IP packets toward their destinations.

A wireless access point connects wireless devices to a network; a modem handles the signalling/modulation required by a particular access link. A consumer box may combine several roles.

A hub repeats signals to its other ports rather than learning destinations, making unnecessary sharing of traffic/capacity more likely. Physical media also matter: copper, fibre and radio have different range, interference, installation and mobility constraints.

Pick the characteristic connected to the scenario, rather than treating wireless as automatically slower regardless of equipment.

Client-server and peer-to-peer

In client-server systems, a server provides a service requested by clients, enabling centralised permissions, backups and consistent records. A failure/overload can affect many clients unless redundancy is provided; administration and infrastructure cost resources.

The server can be dedicated or a process on a machine with other roles.

In peer-to-peer systems, peers can both request and provide resources, distributing work and reducing dependence on one dedicated server. Resource availability and administration depend on participating peers, and consistent access controls/backups can be harder.

Peer-to-peer does not mean there are no protocols or that every system lacks any central discovery service.

Threats and layered controls

Threats include unauthorised access, interception, malware, spoofing/phishing and denial of service. A firewall filters traffic according to rules such as source/destination, ports and connection state; permitted malicious traffic can still pass.

A proxy acts as an intermediary and may filter requests, cache content or mediate access. Neither makes a compromised endpoint safe.

Encryption protects confidentiality of transmitted data; authenticated secure protocols also help detect tampering and establish who is communicating. HTTPS normally protects the browser-to-server connection through TLS. Encryption does not stop a user giving credentials to a fraudulent site or malware reading plaintext at an endpoint.

Authentication, least-privilege permissions, updates, backups, segmentation and monitoring address different risks. Tie each control to a threat and mention a relevant limitation.

Original A-Level practice

6 original questions total 21 marks. Attempt each before opening the independently written indicative marking guidance.