Port 661 is the official IANA-assigned port for the Host Access Protocol (HAP), a network layer protocol designed in the 1980s for satellite-based wide-area networks. While it still holds its official assignment, HAP is largely obsolete in modern networking.
What is HAP?
HAP stands for Host Access Protocol. It's a network layer (OSI Layer 3) protocol that manages communication between hosts and specialized network packet switch nodes called Wideband Packet Switches (WPS).1
The protocol emerged from DARPA's Wideband Packet Satellite Network (WBNET) project, designed to solve a specific problem: how do you efficiently move data through satellite links and wide-area networks when bandwidth is precious and delays are significant?
How HAP Works
HAP operates as a full-duplex protocol that supports two types of communication:1
Datagram messages — Variable-length, unsequenced packets sent without bandwidth guarantees. Send it and hope it arrives. This mode is unreliable by design.
Stream messages — Pre-allocated bandwidth with guaranteed resources and minimum delay. The network reserves capacity for your traffic before you send it.
The protocol uses 16-bit logical addresses and includes flow control mechanisms where the receiver can accept or refuse messages. Resource allocation happens through a Service Agent that manages stream creation, multicast groups, and network resources.
The Satellite Era
HAP was born in an era when "wide-area network" often meant "satellite link." The protocol was designed for networks where:
- Bandwidth was expensive and had to be carefully allocated
- Round-trip delays were measured in hundreds of milliseconds
- You needed both guaranteed streams (for real-time data) and best-effort datagrams (for everything else)
RFC 1221, published in April 1991, documents version 2 of the specification.1 By then, terrestrial fiber networks were already making satellite-based protocols less relevant.
Port Assignment
IANA officially assigns port 661 to HAP on both TCP and UDP.2 This is a well-known port (in the 0-1023 range), reserved by IANA for system services.
The assignment remains in the registry, a historical marker of protocols that once mattered.
Why You Rarely See It
HAP is essentially obsolete. Modern networks don't use Wideband Packet Switches. The problems HAP solved—efficient satellite communication, guaranteed bandwidth allocation, managing scarce wide-area resources—have been addressed by newer protocols and vastly improved infrastructure.
If you see traffic on port 661 today, it's worth investigating. It could be:
- Legacy equipment still running ancient protocols (unlikely)
- Misconfigured software attempting to use an unrelated service on this port
- Malware (the NokNok trojan, among others, has been observed using port 661)3
Checking What's Listening
To see if anything is listening on port 661 on your system:
Linux/Mac:
Windows:
If something is listening and you don't recognize it, investigate. Legitimate HAP traffic in 2026 is rare enough to warrant scrutiny.
Why Unassigned Ports Matter
Port 661 isn't technically unassigned—it has HAP. But functionally, it might as well be. The protocol is extinct.
This matters because the IANA port registry is not just a list of active services. It's a historical record. Every assigned port represents someone solving a problem, building infrastructure, creating standards. Some of those solutions survived. Many didn't.
Port 661 is a fossil. A reminder that the Internet's nervous system still carries traces of networks designed for satellites we no longer use, protocols that made sense when bandwidth cost dollars per kilobit, and solutions to problems that have been solved better since.
The registry preserves these assignments because changing them creates risk—someone might still depend on them. So HAP remains, officially assigned to port 661, waiting for traffic that will probably never come.
Related Ports
- Port 162 — SNMP trap, for network management alerts (a protocol that actually survived)
- Port 520 — RIP (Routing Information Protocol), another protocol from the early Internet that's largely been replaced
- Port 1900 — SSDP (Simple Service Discovery Protocol), a modern example of how service discovery evolved
Frequently Asked Questions
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