Port 423 is officially assigned to opc-job-start, the IBM Operations Planning and Control Start service.1 This is a port that belongs to the world of mainframes and enterprise workload scheduling—a world most people never see.
What It Does
Port 423 carries signals that trigger batch jobs in IBM's Operations Planning and Control system (now called IBM Workload Scheduler for z/OS).2 When a scheduled job needs to start—payroll processing, database maintenance, report generation—port 423 is how the scheduler tells the mainframe to begin.
This is enterprise automation from the era when "Operations Planning and Control" meant something physical: rooms full of operators, scheduled jobs running overnight, careful coordination of system resources. Port 423 is a protocol artifact from that world.
The Context
Port 423 sits in the well-known ports range (0-1023), which means it was assigned by IANA and is considered part of the Internet's official infrastructure.3 This is somewhat remarkable for such a specialized, vendor-specific service. Most IBM-specific protocols live in higher port ranges. Port 423's placement here suggests it was assigned early, when the well-known range was less crowded and assignments were more freely given.
The service uses both TCP and UDP on port 423, though TCP is more common for this type of control protocol.4
What OPC Actually Is
IBM Operations Planning and Control (OPC) was—and in its modern incarnation, still is—a comprehensive workload scheduling system for mainframes.5 It manages dependencies between jobs, handles resource allocation, processes scheduling rules, and coordinates the execution of batch workloads across enterprise systems.
Think of it as a sophisticated alarm clock and task manager for computers that run a company's critical business processes. When your paycheck appears in your bank account on the 15th of the month, there's a decent chance somewhere in that chain of events, an OPC system told a mainframe to start the payroll job. Port 423 might have carried that signal.
Security Considerations
Port 423 is not a common attack target. It's too specialized, too tied to IBM mainframe environments, too obscure to most attackers.6 If you're not running IBM Workload Scheduler, you shouldn't see traffic on port 423.
If you do see unexpected traffic on this port, it's worth investigating—not because port 423 itself is dangerous, but because someone might be probing for IBM mainframe systems on your network.
How to Check What's Listening
On Linux or macOS:
On Windows:
If nothing is listening and you're not running IBM mainframe software, that's expected. Port 423 is a specialized tool for a specialized job.
Why This Port Matters
Port 423 represents a category of ports that most people never encounter: vendor-specific, enterprise-focused, carrying protocols that run invisible infrastructure. You will probably never personally use port 423. But somewhere, right now, it's telling a mainframe to start processing the transactions that run part of the world's financial system, or healthcare records, or airline reservations.
The Internet's port system includes room for these quiet, specialized protocols alongside the famous ones like HTTP and SSH. Port 423 is a reminder that beneath the web browsers and apps, there's a deeper layer of industrial-strength computing that just works, decade after decade, processing the batch jobs that keep enterprises running.
Related Ports
- Port 111 - ONC RPC (Sun Remote Procedure Call), another enterprise coordination protocol
- Port 135 - Microsoft EPMAP (endpoint mapper), similar role in Windows enterprise environments
- Port 530 - RPC (Remote Procedure Call), general-purpose remote procedure coordination
Frequently Asked Questions
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