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Port 321 (pip)

IANA assigns port 321 to pip: PIP.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
pip TCP PIP Gordon Mohr -
pip UDP PIP Gordon Mohr -

Check what is listening on port 321

Linux: is anything listening on TCP 321?
sudo ss -tlnp 'sport = :321'
Linux: UDP 321
sudo ss -ulnp 'sport = :321'
Linux / macOS: which process owns the port
sudo lsof -nP -i :321
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':321 '
Windows (Command Prompt)
netstat -ano | findstr :321
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 321 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 321
From Windows
Test-NetConnection HOST -Port 321
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 321

Block port 321 in the firewall

ufw
sudo ufw deny 321/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 321 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 321 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 321' -Direction Inbound -Protocol TCP -LocalPort 321 -Action Block
ufw
sudo ufw deny 321/udp
iptables
sudo iptables -A INPUT -p udp --dport 321 -j DROP
nftables
sudo nft add rule inet filter input udp dport 321 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 321' -Direction Inbound -Protocol UDP -LocalPort 321 -Action Block

Related ports

1321pip319ptp-event320ptp-general322rtsps323rpki-rtr

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