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Port 204 (at-echo)

IANA assigns port 204 to at-echo: AppleTalk Echo.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
at-echo TCP AppleTalk Echo - -
at-echo UDP AppleTalk Echo - -

Check what is listening on port 204

Linux: is anything listening on TCP 204?
sudo ss -tlnp 'sport = :204'
Linux: UDP 204
sudo ss -ulnp 'sport = :204'
Linux / macOS: which process owns the port
sudo lsof -nP -i :204
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':204 '
Windows (Command Prompt)
netstat -ano | findstr :204
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 204 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 204
From Windows
Test-NetConnection HOST -Port 204
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 204

Block port 204 in the firewall

ufw
sudo ufw deny 204/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 204 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 204 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 204' -Direction Inbound -Protocol TCP -LocalPort 204 -Action Block
ufw
sudo ufw deny 204/udp
iptables
sudo iptables -A INPUT -p udp --dport 204 -j DROP
nftables
sudo nft add rule inet filter input udp dport 204 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 204' -Direction Inbound -Protocol UDP -LocalPort 204 -Action Block

Related ports

202at-nbp203at-3205at-5206at-zis

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