Why Overlay Technologies Like Cloud Firewalls & ZTNA Are Beating Traditional Networking**

Networking is becoming simpler for users… but far more abstract beneath the surface.
Cloud, virtualization, SDN, SASE, ZTNA, and cloud firewalls all depend on one idea:

Overlay networking.

To understand why these modern technologies are winning, you only need to understand two concepts:

• Underlay = the physical roads

• Overlay = the virtual vehicles

Let's break it down with simple visuals.

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1. What Is an Underlay?

Underlay = the physical network.

It includes:

• Cables

• Switches

• Routers

• Physical routing protocols (OSPF, BGP, etc.)

• Physical IP addressing

Think of it like the physical road network beneath a city.

      ┌───────────┐       ┌───────────┐
      │  Switch    │-------│  Router   │
      └───────────┘       └───────────┘
             |                   |
      ┌───────────┐       ┌───────────┐
      │  Cable     │-------│  Switch   │
      └───────────┘       └───────────┘

If the underlay breaks, nothing works.
But the underlay does not decide:

• who can talk to whom

• how traffic is segmented

• how firewall policies are applied

• how security is enforced

It only forwards packets.

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2. What Is an Overlay?

Overlay = a virtual network built on top of the underlay.

Think of it like Uber, Ola, or Google Maps running over the same physical roads—but controlling the route, access, identity, etc. through software.

   ┌─────────────────────────── Overlay (Virtual) ──────────────────────────┐
   │                                                                         │
   │      Virtual Firewalls      VXLAN      ZTNA      SD-WAN      SASE       │
   │                                                                         │
   └─────────────────────────────────────────────────────────────────────────┘

   ┌──────────────────────── Underlay (Physical) ───────────────────────────┐
   │ Switches │ Routers │ Fiber │ Cables │ Routing Protocols │ Connectivity │
   └────────────────────────────────────────────────────────────────────────┘

Overlays don’t care about physical location.

They create:

• Virtual networks

• Virtual firewalls

• Virtual tunnels

• Virtual topologies

• Virtual segmentation

This is why cloud networking works anywhere.

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3. Underlay vs Overlay in One Simple Diagram

                          ┌─────────────────────────┐
                          │       Overlay           │
                          │  (Policies, Security,   │
                          │   Routing, Segmentation)│
                          └──────────┬──────────────┘
                                     │ VXLAN/GRE/IPsec/etc.
                          ┌──────────┴──────────────┐
                          │       Underlay           │
                          │ (Switches, Cables, IPs) │
                          └──────────────────────────┘

Overlay = logic
Underlay = plumbing

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4. Why Overlays Are Winning

Modern networking challenges can’t be solved by physical networks alone.

Traditional networking was limited by:

• location

• physical hardware

• VLAN boundaries

• firewall choke points

• static perimeters

The world changed:

• Apps are global

• Users work remotely

• Workloads run in multiple clouds

• Microservices scale dynamically

• Zero Trust demanded identity-aware security

The underlay can’t solve any of this.

Overlays solve all of it.

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5. Cloud Firewall vs Traditional Firewall

Traditional Firewall (Old Way)

          ┌─────────────────────────────────┐
          │        Perimeter Firewall       │
          └───────────────┬─────────────────┘
                          LAN
                   (Inside the office)

Challenges:

• Must sit at a fixed physical location

• All traffic must pass through it

• Remote workers need VPN

• Limited visibility beyond perimeter

• Hardware failures = outage

• Scaling requires more boxes

Cloud Firewall (Overlay Way)

     ┌───────────────────────────────────────────┐
     │          Cloud Firewall Overlay           │
     │  (SASE / FWaaS / NGFW / Identity-Aware)   │
     └────────────┬────────────┬────────────┬────┘
                  │            │            │
              Branch        Remote      Cloud VMs
              Office        Users       & Apps

Benefits:

• Not tied to any location

• Policies follow the user, not the network

• Ingress/egress can happen globally

• No hardware to maintain

• Auto-scales

• Integrates with identity, device posture, risk signals

• Perfect for multi-cloud

Cloud firewalls live in the overlay world.

That's why they're winning.

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6. ZTNA vs VPN

VPN (Old Underlay-Dependent Model)

Remote User → VPN → Inside Network (Full Access)

Problems:

• User becomes part of the internal subnet

• High lateral movement risk

• Slow (all traffic hairpins)

• Works poorly for SaaS

• Relies on IP-based trust

ZTNA (Overlay Model with Identity)

Remote User
     │
     ▼
Identity + Device Check + Policy
     │
     ▼
Allowed App Only (Not the whole network)

        ┌─────────────────────────────┐
        │           ZTNA              │
        │   (App-level connectivity)  │
        └────────┬──────────┬────────┘
                 │          │
             App A       App B

ZTNA uses an overlay to create per-app tunnels, not full network access.

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7. Why Overlays Beat Traditional Networking

1. Identity > IP Address

Overlays use:

• identity

• user

• device posture

• application

• risk signals

Traditional firewalls use:

• IP

• port

• protocol

Identity-based security is dramatically stronger.

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2. Location No Longer Matters

Apps, users, and workloads move.
Overlays adapt within seconds.

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3. Infinite Scalability

Underlay has physical limits.
Overlay scales with compute.

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4. Multi-Cloud Requires Overlays

AWS, Azure, and GCP cannot stretch VLANs.
But overlays (VXLAN, Geneve, SD-WAN, ZTNA tunnels) work everywhere.

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5. Zero Trust Is Only Possible in Overlay

Because policies follow the identity, not the cable.

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