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Port 3521 (mc3ss)

IANA assigns port 3521 to mc3ss: Telequip Labs MC3SS.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
mc3ss TCP Telequip Labs MC3SS Michael Sparks -
mc3ss UDP Telequip Labs MC3SS Michael Sparks -

Registry dates for this port: registered 2002-06.

Check what is listening on port 3521

Linux: is anything listening on TCP 3521?
sudo ss -tlnp 'sport = :3521'
Linux: UDP 3521
sudo ss -ulnp 'sport = :3521'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3521
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3521 '
Windows (Command Prompt)
netstat -ano | findstr :3521
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3521 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3521
From Windows
Test-NetConnection HOST -Port 3521
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3521

Block port 3521 in the firewall

ufw
sudo ufw deny 3521/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3521 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3521 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3521' -Direction Inbound -Protocol TCP -LocalPort 3521 -Action Block
ufw
sudo ufw deny 3521/udp
iptables
sudo iptables -A INPUT -p udp --dport 3521 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3521 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3521' -Direction Inbound -Protocol UDP -LocalPort 3521 -Action Block

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All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.