Windows provides several built-in command line utilities for checking remote port accessibility. The most commonly used tools are:

1. Telnet (needs to be enabled)
2. Test-NetConnection (PowerShell)
3. PortQry.exe (downloadable from Microsoft)
4. Nmap (third-party but powerful)

First ensure Telnet Client is enabled:

dism /online /Enable-Feature /FeatureName:TelnetClient

Basic syntax for port testing:

telnet [hostname or IP] [port]

Example checking web server on port 80:

telnet example.com 80

A blank screen or server response indicates success, while "Could not open connection" means failure.

More modern approach with detailed output:

Test-NetConnection -ComputerName example.com -Port 443

Sample output structure:

ComputerName     : example.com
RemoteAddress    : 93.184.216.34
RemotePort       : 443
InterfaceAlias   : Ethernet
SourceAddress    : 192.168.1.100
TcpTestSucceeded : True

For batch testing multiple ports:

$ports = 80,443,22,3389
foreach ($port in $ports) {
    Test-NetConnection -ComputerName example.com -Port $port -InformationLevel Quiet
}

Checking UDP ports (requires PortQry):

portqry -n example.com -e 53 -p UDP

If connections fail:

• Verify remote firewall rules
• Check local firewall settings
• Confirm service is running on remote machine
• Validate network routing between hosts

Basic Nmap command for port scan:

nmap -Pn -p 80,443 example.com

This provides more detailed information including service versions.

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When troubleshooting network connectivity or configuring firewalls, checking if a remote port is open is a common task for Windows administrators and developers. This article explores several command-line methods to test port accessibility.

The simplest way to test a remote port is using the built-in telnet client:

telnet [remote_host] [port]

Example:

telnet example.com 80

If the port is open, you'll see either a blank screen or the service banner. If closed, you'll get a connection error.

For more detailed information, PowerShell's Test-NetConnection cmdlet is powerful:

Test-NetConnection -ComputerName [remote_host] -Port [port]

Example:

Test-NetConnection -ComputerName example.com -Port 443

This provides a detailed output including TCP test success, latency, and source/destination addresses.

Microsoft's PortQry provides advanced port scanning capabilities:

PortQry.exe -n [remote_host] -e [port] -p [TCP/UDP]

Example:

PortQry.exe -n example.com -e 3389 -p TCP

This tool can identify filtered ports and provides more detailed status information than basic telnet.

For programmatic checking, you can use this PowerShell script:

$tcp = New-Object System.Net.Sockets.TcpClient
try {
    $tcp.Connect("example.com", 80)
    Write-Host "Port open"
} catch {
    Write-Host "Port closed"
} finally {
    $tcp.Close()
}

Other useful tools include:

• Nmap (requires installation)
• PsPing (from Sysinternals)
• Test-Connection in PowerShell

Remember that a "closed" port might mean:

• The port is actually closed
• A firewall is blocking the connection
• The service isn't running
• Network connectivity issues exist