Without VPN:
Your Device → ISP → Internet → Website
(ISP sees destination, website sees your IP)

With VPN:
Your Device → Encrypted Tunnel → VPN Server → Internet → Website
(ISP sees VPN server only, website sees VPN IP, VPN provider sees everything)

Office A ←── VPN Tunnel ──→ Office B
(Both networks appear as one private network)

Protocols: IPSec, GRE over IPSec, WireGuard

# /etc/wireguard/wg0.conf (Server)
[Interface]
PrivateKey = SERVER_PRIVATE_KEY
Address = 10.0.0.1/24
ListenPort = 51820
PostUp = iptables -A FORWARD -i wg0 -j ACCEPT; iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
PostDown = iptables -D FORWARD -i wg0 -j ACCEPT; iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE

[Peer]
PublicKey = CLIENT_PUBLIC_KEY
AllowedIPs = 10.0.0.2/32

# Client config
[Interface]
PrivateKey = CLIENT_PRIVATE_KEY
Address = 10.0.0.2/24
DNS = 1.1.1.1

[Peer]
PublicKey = SERVER_PUBLIC_KEY
Endpoint = server-ip:51820
AllowedIPs = 0.0.0.0/0        # Route all traffic through VPN
PersistentKeepalive = 25

# Nginx as reverse proxy
server {
    listen 443 ssl;
    server_name app.example.com;

    ssl_certificate /etc/letsencrypt/live/app.example.com/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/app.example.com/privkey.pem;

    location / {
        proxy_pass http://localhost:3000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

Your Device
    │ (encrypted with 3 layers)
    ↓
Guard Node (Entry) — knows your IP, not destination
    │ (decrypts outer layer)
    ↓
Middle Node — knows neither you nor destination
    │ (decrypts middle layer)
    ↓
Exit Node — knows destination, not you
    │
    ↓
Destination Website

Private Network                          Internet
192.168.1.10  ─┐                    ┌── 8.8.8.8 (Google DNS)
192.168.1.11  ─┤── Router (NAT) ───┤── 1.1.1.1 (Cloudflare)
192.168.1.12  ─┘  Public IP: 1.2.3.4  └── 93.184.216.34 (example.com)

# iptables port forwarding: external port 8080 → internal server 192.168.1.100:80
iptables -t nat -A PREROUTING -p tcp --dport 8080 -j DNAT --to-destination 192.168.1.100:80
iptables -A FORWARD -p tcp -d 192.168.1.100 --dport 80 -j ACCEPT

Full tunnel:    All traffic → VPN → Internet
Split tunnel:   Corporate traffic → VPN; Personal traffic → Direct ISP

Benefits of split tunnel:
- Faster for non-corporate traffic
- Less load on VPN server
- Streaming services work without VPN IP blocks

Risk:
- Personal browsing bypasses corporate monitoring/protection