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Port 8501 (cmtp-mgt, cmtp-av)

IANA assigns port 8501 to cmtp-mgt, cmtp-av: CYTEL Message Transfer Management.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
cmtp-mgt TCP CYTEL Message Transfer Management - -
cmtp-av UDP CYTEL Message Transfer Audio and Video Frank Jakel -

Registry dates for this port: registered 2011-04-29.

Check what is listening on port 8501

Linux: is anything listening on TCP 8501?
sudo ss -tlnp 'sport = :8501'
Linux: UDP 8501
sudo ss -ulnp 'sport = :8501'
Linux / macOS: which process owns the port
sudo lsof -nP -i :8501
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':8501 '
Windows (Command Prompt)
netstat -ano | findstr :8501
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 8501 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 8501
From Windows
Test-NetConnection HOST -Port 8501
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 8501

Block port 8501 in the firewall

ufw
sudo ufw deny 8501/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 8501 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 8501 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 8501' -Direction Inbound -Protocol TCP -LocalPort 8501 -Action Block
ufw
sudo ufw deny 8501/udp
iptables
sudo iptables -A INPUT -p udp --dport 8501 -j DROP
nftables
sudo nft add rule inet filter input udp dport 8501 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 8501' -Direction Inbound -Protocol UDP -LocalPort 8501 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.