Port 243 was assigned to "Survey Measurement" (sur-meas) in the early ARPANET era. It was part of the Network Measurement Group's efforts to understand how the first packet-switched network actually performed in practice.
What Survey Measurement Did
Starting in late 1971, a program called SURVEY monitored ARPANET host behavior—whether hosts were reachable, how they responded, what services were available. Port 243 carried this measurement data between hosts participating in the survey project.
The protocol was experimental. The ARPANET was the first network of its kind, and nobody knew how to measure its performance systematically. Survey Measurement was one of the first attempts to turn network performance from guesswork into data.1
The Story: Measuring What Nobody Had Measured Before
In March 1972, Vint Cerf organized a meeting at MIT to form the Network Measurement Group (NMG).2 The goal was straightforward: figure out how to measure the ARPANET's performance.
The SURVEY program had already been running since late 1971, collecting information about host availability and network behavior. But there was a problem—storage was expensive, and they couldn't keep all the data. The program saved short daily summaries and discarded the detailed measurements.3
In 1973, Computer Corporation of America (CCA) and MIT's Project MAC started a joint project to transmit SURVEY data to CCA's "datacomputer" for storage and analysis.4 This was revolutionary: storing detailed network measurements so researchers could analyze patterns over time.
Port 243 was part of this infrastructure—a designated channel for measurement data to flow through the network being measured.
Why This Matters
Survey Measurement represents the Internet's first attempt at introspection. The ARPANET team didn't just build a network—they immediately tried to understand how it behaved.
This impulse, the need to measure and monitor network performance, became fundamental to how the Internet works. Every network monitoring tool, every performance dashboard, every uptime metric traces its lineage back to these early experiments.
They were measuring the network with the network. Using port 243 to understand how all the other ports were performing.
Current Status
Port 243 is officially assigned to "sur-meas" in the IANA registry,5 but the original ARPANET Survey Measurement protocol is no longer in active use. The port remains reserved, a historical marker of the Internet's first systematic measurement efforts.
Modern network measurement uses different protocols and ports—SNMP, NetFlow, IPFIX—but the principle is the same: you can't manage what you don't measure.
Security Considerations
Port 243 is rarely seen in modern networks. If you see traffic on this port, it's worth investigating—it's unlikely to be legitimate Survey Measurement protocol (which hasn't been used in decades) and could indicate:
- Misconfigured software using an unusual port
- Custom applications using unassigned ports
- Potentially malicious activity hiding in uncommon port space
Always verify unexpected traffic on well-known ports that lack current service assignments.
How to Check What's Listening
To see if anything is listening on port 243 on your system:
Linux/Mac:
Windows:
If you find something listening on port 243, investigate what application opened it. Legitimate modern software shouldn't need this port.
The Unassigned Port Space
Port 243 sits in an interesting category: officially assigned to a historical protocol that's no longer used. The well-known port range (0-1023) contains many such ports—reserved for services that either evolved into different protocols, moved to different ports, or faded into obsolescence.
These ghost ports are part of the Internet's archaeological record. They document what people tried, what worked, what didn't, and what got replaced by something better.
Related Ports
- Port 241: NCP Measurement—another experimental ARPANET measurement protocol
- Port 161/162: SNMP—the modern standard for network monitoring
- Port 2055: NetFlow—Cisco's network flow monitoring protocol
- Port 4739: IPFIX—the standardized IP Flow Information Export protocol
Frequently Asked Questions
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