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Port 203 (at-3)

IANA assigns port 203 to at-3: AppleTalk Unused.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
at-3 TCP AppleTalk Unused - -
at-3 UDP AppleTalk Unused - -

Check what is listening on port 203

Linux: is anything listening on TCP 203?
sudo ss -tlnp 'sport = :203'
Linux: UDP 203
sudo ss -ulnp 'sport = :203'
Linux / macOS: which process owns the port
sudo lsof -nP -i :203
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':203 '
Windows (Command Prompt)
netstat -ano | findstr :203
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 203 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 203
From Windows
Test-NetConnection HOST -Port 203
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 203

Block port 203 in the firewall

ufw
sudo ufw deny 203/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 203 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 203 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 203' -Direction Inbound -Protocol TCP -LocalPort 203 -Action Block
ufw
sudo ufw deny 203/udp
iptables
sudo iptables -A INPUT -p udp --dport 203 -j DROP
nftables
sudo nft add rule inet filter input udp dport 203 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 203' -Direction Inbound -Protocol UDP -LocalPort 203 -Action Block

Related ports

201at-rtmp202at-nbp204at-echo205at-5

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