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Port 3933 (plbserve-port)

IANA assigns port 3933 to plbserve-port: PL/B App Server User Port.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
plbserve-port TCP PL/B App Server User Port Edward R Boedecker -
plbserve-port UDP PL/B App Server User Port Edward R Boedecker -

Registry dates for this port: registered 2003-11.

Check what is listening on port 3933

Linux: is anything listening on TCP 3933?
sudo ss -tlnp 'sport = :3933'
Linux: UDP 3933
sudo ss -ulnp 'sport = :3933'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3933
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3933 '
Windows (Command Prompt)
netstat -ano | findstr :3933
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3933 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3933
From Windows
Test-NetConnection HOST -Port 3933
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3933

Block port 3933 in the firewall

ufw
sudo ufw deny 3933/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3933 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3933 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3933' -Direction Inbound -Protocol TCP -LocalPort 3933 -Action Block
ufw
sudo ufw deny 3933/udp
iptables
sudo iptables -A INPUT -p udp --dport 3933 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3933 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3933' -Direction Inbound -Protocol UDP -LocalPort 3933 -Action Block

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