App-ID, Application Default, or port-based — when to use what
Today my security manager asked me a question.
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By Mrugesh Patel, Senior Network Security Engineer
Originally posted on LinkedIn — Engineer Series · 2026-05-11
Today my security manager asked me a question.
“When should we use App-ID? When should we add Application Default? When should we layer port-based rules on top?”
And it got me thinking — there are probably a lot of engineers and leaders asking the same thing right now.
So let me share how I think about it.
Picture a firewall like security at an airport.
→ Port-based rules — letting people in based on which door they use
→ App-ID — checking who they are and what they’re actually doing
→ Application Default — only letting them through the door they’re supposed to use
That mental model changes how you write every rule.
1️⃣ App-ID — your default approach
App-ID identifies the actual application, regardless of port.
✅ You want real visibility into your traffic
✅ Modern apps don’t stick to standard ports (most don’t)
✅ You’d rather control behavior than just open a port
Instead of “Allow port 443” — say “Allow Microsoft Teams, Office 365, web-browsing.”
You’re controlling what flows. Not just where.
2️⃣ Application Default — your safety net
Allow the application only on the ports it’s supposed to use.
→ HTTPS → only port 443
→ DNS → only port 53
It prevents apps from sneaking through unexpected ports. It reduces your attack surface. It’s the cleanest policy you can write.
The catch: it can break apps that genuinely need non-standard ports. Coordinate with app teams before flipping it on.
3️⃣ Port-based on top of App-ID — sparingly
Sometimes you have a legitimate exception. A business app on port 8443. A legacy system that needs custom port handling.
That’s when you layer a specific port onto the App-ID rule. Not as a default. As a documented exception.
The risk picture:
🔴 Port-based only → high risk. No visibility. Easy to bypass. 🟡 App-ID on any port → medium. App allowed on unexpected ports. 🟢 App-ID + Application Default → low. The gold standard. 🟠 App-ID + custom ports → medium. Controlled, but introduces gaps if not reviewed.
The shift that changes everything:
Most firewall requests come in like:
“Open this IP and this port.”
The right question is:
“What application is this — and should we trust it?”
That shift is what separates firewall configuration from real security engineering.
If you’re still relying heavily on port-based rules in 2026, you’re locking your front door and leaving the windows open.
App-ID exists for a reason. Use it.
How does your team approach this — App-ID first, or still port-driven?
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