What This Port Is
Port 2587 belongs to the registered ports range (1024–49151). IANA has officially assigned it to MASC — the Multicast Address-Set Claim Protocol — on both TCP and UDP. So it's not truly unassigned. It's just quiet.
MASC was defined in RFC 2909 in September 2000. The idea was legitimate: if multiple organizations are all running multicast networks, how do they avoid stepping on each other's multicast address space? MASC proposed a hierarchical claim system — routers would negotiate address prefixes across domains, claiming space from a parent node and delegating subsets to children. Port 2587 was the door you knocked on to initiate that negotiation.1
It was a thoughtful solution to a real coordination problem.
Why You've Never Heard of It
Multicast routing at Internet scale never happened. The Multicast Backbone (MBone) experiments of the 1990s were promising but never became infrastructure. ISPs were reluctant to carry multicast traffic, the routing overhead was substantial, and the deployment complexity was high. Without widespread inter-domain multicast, MASC had nothing to coordinate. RFC 2909 became a historical artifact, and port 2587 became one of those reserved addresses that sits in IANA's registry with no active tenants.2
This is not unusual. The registered ports range contains hundreds of entries for protocols that were standardized, implemented in a few places, and then quietly superseded or abandoned as the Internet evolved differently than its architects expected.
What You'll Actually Find Here
In practice, port 2587 carries no standard traffic. If you see activity on it:
- Scanning probes — automated scanners sweep large port ranges looking for open services; 2587 gets caught in the net
- Malware or custom software — some applications pick obscure registered ports precisely because they're unrecognized by firewalls and monitoring tools
- Nothing — the most common answer
Checking What's Listening
If port 2587 shows up open on a machine you control, find out what owns it:
Linux / macOS:
Windows:
The process ID in the output maps to an entry in Task Manager or tasklist. If you don't recognize it, investigate before leaving it open.
Why Unassigned (and Quiet) Ports Matter
The registered ports range exists so that software developers can stake out a port number without colliding with something else. Most of those registrations represent real protocols — some widely deployed, some never leaving the lab. Port 2587 is a reminder that the port registry is aspirational as much as descriptive. It records what protocols intended to use a port, not necessarily what's happening there today.
Firewalls that block "unknown" ports by default are implicitly exploiting this gap — most registered ports carry no traffic on most networks, so denying them by default costs almost nothing and eliminates a lot of noise.
Related Ports
- Port 587 — SMTP mail submission (the widely deployed port that 2587 superficially resembles in number — no actual relationship)
- Port 4604 — Also associated with multicast address allocation research
- Port 2090 — Another registered port from the same era of multicast experimentation
Frequently Asked Questions
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