Port 203 AT-3
Port 203 is the IANA-assigned at-3 port for AppleTalk Unused (TCP / UDP).
What is port 203 used for?
Port 203 is assigned to at-3 and uses TCP / UDP. The IANA registry describes the service as “AppleTalk Unused”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach at-3 connects to port 203. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 203 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Source: IANA Service Name and Transport Protocol Port Number Registry.
Check and block port 203
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :203'
sudo ss -ulpn 'sport = :203'Check on Windows
Get-NetTCPConnection -LocalPort 203 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 203 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 203/tcp
sudo ufw deny 203/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 203 -j DROP
sudo iptables -A INPUT -p udp --dport 203 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 203 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Where port 203 is usually used
Most traffic on port 203 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Core infrastructure that clients expect to find at its default number.
- Firewall reviews that baseline which classic services are reachable.
- Troubleshooting connectivity when an application assumes the standard port.
Probe the port
Confirm something is listening before you debug the client. Replace 127.0.0.1 with the host you are testing.
nc -vz 127.0.0.1 203Common problems with port 203
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 203. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :203' or sudo lsof -i :203. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 203 at a glance
| Port number | 203 |
|---|---|
| Service name | at-3 |
| Transport protocol | TCP / UDP |
| Category | Core network services |
| Range | Well-known port (0–1023) |
| Risk level | Standard/Low |
| Registered assignee | Not listed |
| Registry description | AppleTalk Unused |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 203 used for?
Port 203 (AT-3) uses TCP / UDP. AppleTalk Unused.
How do I check if port 203 is open?
On Linux run `sudo ss -tlpn 'sport = :203'` and `sudo ss -ulpn 'sport = :203'`. On Windows run `Get-NetTCPConnection -LocalPort 203 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 203 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 203; empty output usually means nothing is bound to it.
How do I block port 203?
With UFW run `sudo ufw deny 203/tcp` and `sudo ufw deny 203/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 203 -j DROP` and `sudo iptables -A INPUT -p udp --dport 203 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 203 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy