New answer, powershell

TCP

Get-Process -Id (Get-NetTCPConnection -LocalPort YourPortNumberHere).OwningProcess

UDP

Get-Process -Id (Get-NetUDPEndpoint -LocalPort YourPortNumberHere).OwningProcess

Old answer, cmd

 C:\> netstat -a -b

(Add -n to stop it trying to resolve hostnames, which will make it a lot faster.)

Note Dane's recommendation for TCPView. It looks very useful!

-a Displays all connections and listening ports.

-b Displays the executable involved in creating each connection or listening port. In some cases well-known executables host multiple independent components, and in these cases the sequence of components involved in creating the connection or listening port is displayed. In this case the executable name is in [] at the bottom, on top is the component it called, and so forth until TCP/IP was reached. Note that this option can be time-consuming and will fail unless you have sufficient permissions.

-n Displays addresses and port numbers in numerical form.

-o Displays the owning process ID associated with each connection.

For Windows:

netstat -aon | find /i "listening"

Use TCPView if you want a GUI for this. It's the old Sysinternals application that Microsoft bought out.

The -b switch mentioned in most answers requires you to have administrative privileges on the machine. You don't really need elevated rights to get the process name!

Find the pid of the process running in the port number (e.g., 8080)

netstat -ano | findStr "8080"

Find the process name by pid

tasklist /fi "pid eq 2216"

netstat -aof | findstr :8080 (Change 8080 for any port)

To find out which specific process (PID) is using which port:

netstat -anon | findstr 1234

Where 1234 is the PID of your process. [Go to Task Manager → Services/Processes tab to find out the PID of your application.]

In case someone need an equivalent for macOS like I did, here is it:

lsof -i tcp:8080

After you get the PID of the process, you can kill it with:

kill -9 <PID>

If you'd like to use a GUI tool to do this there's Sysinternals' TCPView.

1. Open the command prompt - start → Run → cmd, or start menu → All Programs → Accessories → Command Prompt.

2. Type

   netstat -aon | findstr '[port_number]'
   

Replace the [port_number] with the actual port number that you want to check and hit Enter.

3. If the port is being used by any application, then that application’s detail will be shown. The number, which is shown at the last column of the list, is the PID (process ID) of that application. Make note of this.

4. Type

   tasklist | findstr '[PID]'
   

Replace the [PID] with the number from the above step and hit Enter.

5. You’ll be shown the application name that is using your port number.

Netstat:

• -a displays all connection and listening ports
• -b displays executables
• -n stop resolve hostnames (numerical form)

• -o owning process

  netstat -bano | findstr "7002"
  
  netstat -ano > ano.txt 
  

The Currports tool helps to search and filter

A single-line solution that helps me is this one. Just substitute 3000 with your port:

$P = Get-Process -Id (Get-NetTCPConnection -LocalPort 3000).OwningProcess; Stop-Process $P.Id

Edit: Changed kill to Stop-Process for more PowerShell-like language

To find pid who using port 8000

netstat -aon | findstr '8000'

To Kill that Process in windows

taskkill /pid pid /f

where pid is the process id which you get form first command

Programmatically, you need stuff from iphlpapi.h, for example GetTcpTable2(). Structures like MIB_TCP6ROW2 contain the owner PID.

You can also check the reserved ports with the command below. Hyper-V reserve some ports, for instance.

netsh int ipv4 show excludedportrange protocol=tcp

Powershell

if you want to have a good overview, you can use this:

Get-NetTCPConnection -State Listen | Select-Object -Property *, `
    @{'Name' = 'ProcessName';'Expression'={(Get-Process -Id $_.OwningProcess).Name}} `
    | select ProcessName,LocalAddress,LocalPort

than you get a table like this:

ProcessName              LocalAddress  LocalPort
-----------              ------------  ---------
services                 ::                49755
jhi_service              ::1               49673
svchost                  ::                  135
services                 0.0.0.0           49755
spoolsv                  0.0.0.0           49672

for udp it is:

Get-NetUDPEndpoint | Select-Object -Property *, `
   @{'Name' = 'ProcessName';'Expression'={(Get-Process -Id $_.OwningProcess).Name}} `
   | select ProcessName,LocalAddress,LocalPort

Use the below batch script which takes a process name as an argument and gives netstat output for the process.

@echo off
set procName=%1
for /f "tokens=2 delims=," %%F in ('tasklist /nh /fi "imagename eq %1" /fo csv') do call :Foo %%~F
goto End

:Foo
set z=%1
echo netstat for : "%procName%" which had pid "%1"
echo ----------------------------------------------------------------------

netstat -ano |findstr %z%
goto :eof

:End

Based on answers with info and kill, for me it is useful to combine them in one command. And you can run this from cmd to get information about process that listen on given port (example 8080):

for /f "tokens=3 delims=LISTENING" %i  in ('netstat -ano ^| findStr "8080" ^| findStr "["') do @tasklist /nh /fi "pid eq %i"

Or if you want to kill it:

for /f "tokens=3 delims=LISTENING" %i  in ('netstat -ano ^| findStr "8080" ^| findStr "["') do @Taskkill /F /IM %i

You can also put those command into a bat file (they will be slightly different - replace %i for %%i):

File portInfo.bat

for /f "tokens=3 delims=LISTENING" %%i  in (
    'netstat -ano ^| findStr "%1" ^| findStr "["'
) do @tasklist /nh /fi "pid eq %%i"

File portKill.bat

for /f "tokens=3 delims=LISTENING" %%i  in (
    'netstat -ano ^| findStr "%1" ^| findStr "["'
) do @Taskkill /F /IM %%i

Then you from cmd you can do this:

portInfo.bat 8080

or

portKill.bat 8080