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Port 187 (aci)

IANA assigns port 187 to aci: Application Communication Interface.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
aci TCP Application Communication Interface Rick Carlos -
aci UDP Application Communication Interface Rick Carlos -

Check what is listening on port 187

Linux: is anything listening on TCP 187?
sudo ss -tlnp 'sport = :187'
Linux: UDP 187
sudo ss -ulnp 'sport = :187'
Linux / macOS: which process owns the port
sudo lsof -nP -i :187
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':187 '
Windows (Command Prompt)
netstat -ano | findstr :187
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 187 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 187
From Windows
Test-NetConnection HOST -Port 187
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 187

Block port 187 in the firewall

ufw
sudo ufw deny 187/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 187 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 187 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 187' -Direction Inbound -Protocol TCP -LocalPort 187 -Action Block
ufw
sudo ufw deny 187/udp
iptables
sudo iptables -A INPUT -p udp --dport 187 -j DROP
nftables
sudo nft add rule inet filter input udp dport 187 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 187' -Direction Inbound -Protocol UDP -LocalPort 187 -Action Block

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