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// Other services

Port 203: AppleTalk Unused

Port 203 is assigned by IANA to at-3 (AppleTalk Unused).

203

How at-3 works

Other services
client:203at-3response client:203at-3response

The client sends a request to port 203 and the at-3 service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Other services
01 Security

Is port 203 dangerous?

We have no specific security note for port 203. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 203

ServicePortTransportDescriptionReferenceUpdated
at-3 203 TCP, UDP AppleTalk Unused - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 203, and is it reachable?

Linux

sudo ss -ltnup 'sport = :203'      # process listening on 203
sudo lsof -nP -i :203

macOS

sudo lsof -nP -i :203

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 203 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 203
netstat -ano | findstr :203

From another machine

nmap -p 203 -sV <host>
sudo nmap -sU -p 203 <host>
nc -zv <host> 203

Block it in the firewall

sudo ufw deny 203/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 203" -Direction Inbound -LocalPort 203 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 203 used for?

Port 203 is assigned by IANA to at-3 (AppleTalk Unused).

Is port 203 TCP or UDP?

Port 203 is registered for TCP and UDP.

Is it safe to open port 203?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Close the port if nothing on the host needs it.

How do I check if port 203 is open?

On Linux run "ss -ltnup 'sport = :203'", on macOS "lsof -nP -i :203", and on Windows "netstat -ano | findstr :203" to see the local process. From another machine, "nmap -p 203 <host>" shows whether it is reachable.

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