So far we have learned:
- Network Interfaces
- Routing
- Netmasks
- Network Segmentation
We now understand how systems communicate within and between networks.
The next question is:
What if I need access to a network
that I am not physically connected to?
The answer is:
VPN
What Is A VPN?
VPN stands for:
Virtual Private Network
A VPN allows a system to communicate with a remote network as if it were directly connected.
Think of it as:
A secure tunnel
through another network.
Most commonly:
Internet
↓
Encrypted Tunnel
↓
Corporate Network
Why VPNs Exist
Imagine you work from home.
Home PC
needs access to:
Corporate File Server
Without a VPN:
Home PC
X
Corporate Network
No direct connectivity exists.
With a VPN:
Home PC
│
▼
Encrypted Tunnel
│
▼
Corporate Network
The connection becomes possible.
Basic VPN Example
Internet
│
▼
VPN Gateway
│
┌───────────────┼───────────────┐
│ │ │
File Server SQL Server Domain Controller
The VPN gateway becomes your entry point into the network.
What Happens During Connection?
When a VPN connects:
- Authentication occurs
- Encryption is established
- A virtual network interface is created
- Routes are added
Example:
Before:
Ethernet
192.168.1.100
After:
Ethernet
192.168.1.100
VPN
10.10.50.100
A new interface appears.
VPN Interfaces
Most VPN software creates a virtual interface.
Linux:
tun0
Windows:
OpenVPN Adapter
WireGuard Adapter
AnyConnect Adapter
The operating system treats this like a normal network interface.
Routes Matter
The most important thing a VPN usually changes is:
Routing
Suppose:
Corporate Network
10.0.0.0/8
After VPN connection:
10.0.0.0/8
↓
VPN Interface
Traffic destined for corporate systems is sent through the tunnel.
Split Tunnel VPN
Many organizations use:
Split Tunneling
Example:
Corporate Traffic
↓
VPN
Internet Traffic
↓
Home Router
Visualized:
Laptop
┌────┴────┐
│ │
▼ ▼
VPN Internet
10.0.0.0 Everything Else
Only corporate traffic uses the VPN.
Full Tunnel VPN
Some organizations route everything through the VPN.
Example:
Laptop
│
▼
VPN
│
▼
Corporate Firewall
│
▼
Internet
All traffic passes through the corporate network.
Why Companies Do This
Benefits:
Centralized Monitoring
Centralized Filtering
Centralized Logging
The company can inspect traffic before it reaches the Internet.
VPN Encryption
One of the primary benefits of a VPN is encryption.
Without encryption:
Attacker
↓
Can potentially observe traffic (easily readable)
With encryption:
Attacker
↓
Sees encrypted packets (looks like random bytes)
The contents remain protected.
Common VPN Technologies
Examples:
OpenVPN
WireGuard
IPsec
Cisco AnyConnect
Palo Alto GlobalProtect
Different implementations.
Same basic concept.
VPNs In Enterprise Environments
A typical enterprise setup:
Internet
│
▼
VPN Gateway
│
┌────────────┼────────────┐
│ │ │
Workstations Servers Domain Controllers
Remote users connect through the VPN gateway.
Operator Notes
A VPN usually performs three things:
Creates A Tunnel
Creates An Interface
Adds Routes
Whenever you connect to a VPN, inspect:
ip addr
ip route
or
ipconfig
route print
to understand:
- Which interface was created
- Which IP address was assigned
- Which routes were added
For operators, VPNs are fundamentally routing and tunneling technologies. Understanding those two concepts makes VPNs significantly easier to understand and troubleshoot.