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L5.3 — Network Design and Operations. This lecture distinguishes between internet, intranet, and extranet networks, explaining who can access each and how they are secured. It then introduces firewalls as rule-based packet gatekeepers between LAN and WAN, before diving in depth into VPNs — how they work via routing, bridging, and encapsulation, the three main VPN protocols (PPTP, L2TP/IPSec, IKEv2/IPSec), their use cases (secure remote access, privacy, geo-restriction bypass), and their limitations (speed overhead, no end-to-end encryption).

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📋 At a Glance

• [ ] Define the Internet — global public network using TCP/IP; accessible to everyone; security risks include lack of privacy, confidentiality, and integrity
• [ ] Define an Intranet — private mini-internet for an organisation's members only; isolated from public threats via firewalls and VPNs
• [ ] Define an Extranet — like an intranet, but extends controlled access to external partners, suppliers, or clients with restricted permissions
• [ ] Explain what keeps intranet/extranet secure: access controls (role-based), encryption (SSL/TLS), firewalls and VPNs
• [ ] Explain how a firewall works — sits between LAN and WAN, evaluates each packet against a list of rules, forwards if allowed, blocks otherwise
• [ ] Give examples of firewall rules — allow traffic from trusted IPs; block all incoming traffic on port 80
• [ ] State that firewalls block unsolicited inbound connections by default (primary threat is from outside coming in)
• [ ] Define a VPN — Virtual Private Network; establishes an encrypted tunnel to a trusted server; all traffic is encrypted in this channel
• [ ] Trace the VPN packet journey: device encrypts → tunnel to VPN server → VPN server decrypts → forwards to destination → response encrypted back
• [ ] Clarify that VPN is not end-to-end encryption — only encrypts between your device and the VPN server; what happens beyond the server is unknown
• [ ] Explain how VPN achieves network membership via routing (Layer 3 — new virtual router created) and bridging (Layer 2 — device appears inside network)
• [ ] Explain encapsulation — Alice's packet is wrapped in a new container (outer packet), travels through the tunnel, container is removed at destination
• [ ] List the three VPN protocols: PPTP (fast, not secure), L2TP/IPSec (secure, reliable, commonly used), IKEv2/IPSec (very fast, secure, ideal for mobile)
• [ ] List VPN use cases: secure remote access, security on untrusted networks, privacy/anonymity (ISP only sees VPN server), bypass geo-restrictions
• [ ] State VPN limitations: performance (encryption overhead slows speed), complexity/cost (maintaining endpoints), no end-to-end security (unencrypted beyond VPN server)

1. Internet, Intranet, and Extranet