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Port 55 (isi-gl)

IANA assigns port 55 to isi-gl: ISI Graphics Language.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
isi-gl TCP ISI Graphics Language - -
isi-gl UDP ISI Graphics Language - -

Check what is listening on port 55

Linux: is anything listening on TCP 55?
sudo ss -tlnp 'sport = :55'
Linux: UDP 55
sudo ss -ulnp 'sport = :55'
Linux / macOS: which process owns the port
sudo lsof -nP -i :55
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':55 '
Windows (Command Prompt)
netstat -ano | findstr :55
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 55 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 55
From Windows
Test-NetConnection HOST -Port 55
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 55

Block port 55 in the firewall

ufw
sudo ufw deny 55/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 55 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 55 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 55' -Direction Inbound -Protocol TCP -LocalPort 55 -Action Block
ufw
sudo ufw deny 55/udp
iptables
sudo iptables -A INPUT -p udp --dport 55 -j DROP
nftables
sudo nft add rule inet filter input udp dport 55 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 55' -Direction Inbound -Protocol UDP -LocalPort 55 -Action Block

Related ports

53DNS54xns-ch56xns-auth57any private terminal access

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