VPN Fundamentals — IPsec, OpenVPN, WireGuard and the Math That Makes Them Work

Manish Garg
Manish Garg Associate of (ISC)² · RingSafe
Apr 19, 2026
10 min read
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Last updated: May 1, 2026

A VPN tunnels Layer 3 (or Layer 2) traffic over an untrusted network, with confidentiality, integrity, and authentication. The three protocols you need to know are IPsec (the enterprise default), OpenVPN (the legacy SSL VPN), and WireGuard (the modern lightweight default). This module compares the three, walks through phase 1 / phase 2 of IPsec, the cryptographic primitives at play, and how to choose. Module 16 covers ZTNA — the alternative that increasingly replaces traditional VPN.

A VPN is a tunnel: take a packet, encrypt it, wrap it in a new outer packet, ship it over the internet, decrypt at the other side. Easy in concept; the security depends entirely on the cryptographic handshake and the policy you put around it. This module compares the three VPN protocols you will actually meet in 2026 — IPsec for site-to-site, OpenVPN for legacy remote access, WireGuard for everything modern — and demystifies the IPsec phase 1/phase 2 model that confuses every beginner.

What "VPN" actually means in 2026

Three patterns are all called “VPN” but solve different problems. Site-to-site VPN connects two networks (HQ to a branch office, on-prem to a cloud VPC) so hosts on either side can talk as if on the same backbone. Remote-access VPN connects a single user’s laptop to a corporate network so they can reach internal apps. Consumer VPN (NordVPN, ProtonVPN, Surfshark) tunnels traffic through a third-party endpoint to anonymise or geo-shift the user — a fundamentally different threat model from corporate VPN. This module focuses on the first two; consumer VPN is briefly addressed in M16 alongside the ZTNA comparison.

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