LookupPersistentTcpPortReservation function (iphlpapi.h)

The LookupPersistentTcpPortReservation function looks up the token for a persistent TCP port reservation for a consecutive block of TCP ports on the local computer.


IPHLPAPI_DLL_LINKAGE ULONG LookupPersistentTcpPortReservation(
  USHORT   StartPort,
  USHORT   NumberOfPorts,
  PULONG64 Token



The starting TCP port number in network byte order.


The number of TCP port numbers that were reserved.


A pointer to a port reservation token that is returned if the function succeeds.

Return value

If the function succeeds, the return value is NO_ERROR.

If the function fails, the return value is one of the following error codes.

Return code Description
An invalid parameter was passed to the function. This error is returned if zero is passed in the StartPort or NumberOfPorts parameters.
The element was not found. This error is returned if persistent port block specified by the StartPort and NumberOfPorts parameters could not be found.
Use FormatMessage to obtain the message string for the returned error.


The LookupPersistentTcpPortReservation function is defined on Windows Vista and later.

The LookupPersistentTcpPortReservation function is used to lookup the token for a persistent reservation for a block of TCP ports.

A persistent reservation for a block of TCP ports is created by a call to the CreatePersistentTcpPortReservation function. The StartPort or NumberOfPorts parameters passed to the LookupPersistentTcpPortReservation function must match the values used when the persistent reservation for a block of TCP ports was created by the CreatePersistentTcpPortReservation function.

If the LookupPersistentTcpPortReservation function succeeds, the Token parameter returned will point to the token for the persistent port reservation for the block of TCP ports. Note that the token for a given persistent reservation for a block of TCP ports may change each time the system is restarted.

An application can request port assignments from the TCP port reservation by opening a TCP socket, then calling the WSAIoctl function specifying the SIO_ASSOCIATE_PORT_RESERVATION IOCTL and passing the reservation token before issuing a call to the bind function on the socket.


The following example looks up a persistent TCP port reservation and then creates a socket and allocates a port from the port reservation.

#ifndef UNICODE
#define UNICODE


#include <Windows.h.>
#include <winsock2.h>
#include <mstcpip.h>
#include <ws2ipdef.h>
#include <iphlpapi.h>
#include <stdio.h>
#include <stdlib.h>

// Need to link with iphlpapi.lib
#pragma comment(lib, "iphlpapi.lib")

// Need to link with Ws2_32.lib for Winsock functions
#pragma comment(lib, "ws2_32.lib")

int wmain(int argc, WCHAR **argv)  {

    // Declare and initialize variables
    int startPort = 0;         // host byte order
    int numPorts = 0;
    USHORT startPortns = 0;    // Network byte order
    ULONG64 resToken = {0};
    unsigned long status = 0;

    WSADATA wsaData = { 0 };
    int iResult = 0;

    int iFamily = AF_INET;
    int iType = SOCK_STREAM;
    int iProtocol = IPPROTO_TCP;

    DWORD bytesReturned = 0;

    // Note that the sockaddr_in struct works only with AF_INET not AF_INET6
    // An application needs to use the sockaddr_in6 for AF_INET6
    sockaddr_in service; 
    sockaddr_in sockName;
    int nameLen = sizeof(sockName);
    // Validate the parameters
    if (argc != 3) {
        wprintf(L"usage: %s <Starting Port> <Number of Ports>\n", 
        wprintf(L"Look up a persistent TCP port reservation\n");
        wprintf(L"Example usage:\n");
        wprintf(L"   %s 5000 20\n", argv[0]);
        wprintf(L"   where StartPort=5000 NumPorts=20");
        return 1;

    startPort = _wtoi(argv[1]);
    if ( startPort < 0 || startPort> 65535) {
        wprintf(L"Starting point must be either 0 or between 1 and 65,535\n");
        return 1;
    startPortns = htons((USHORT) startPort);

    numPorts = _wtoi(argv[2]);
    if (numPorts < 0) {
        wprintf(L"Number of ports must be a positive number\n");
        return 1;

    status = LookupPersistentTcpPortReservation((USHORT) startPortns, (USHORT) numPorts, &resToken);
    if( status != NO_ERROR )
        wprintf(L"LookupPersistentTcpPortReservation returned error: %ld\n", 
        return 1;

    wprintf(L"LookupPersistentTcpPortReservation call succeeded\n");
    wprintf(L"  Token = %I64d\n", resToken);  
    // Comment out this block if you don't want to create a socket and associate it with the 
    // persistent reservation

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0) {
        wprintf(L"WSAStartup failed with error = %d\n", iResult);
        return 1;

    sock = socket(iFamily, iType, iProtocol);
    if (sock == INVALID_SOCKET)
        wprintf(L"socket function failed with error = %d\n", WSAGetLastError());
    else {
        wprintf(L"socket function succeeded\n");

        iResult =
            WSAIoctl(sock, SIO_ASSOCIATE_PORT_RESERVATION, (LPVOID) & resToken,
                     sizeof (ULONG64), NULL, 0, &bytesReturned, NULL, NULL);
        if (iResult != 0) {
                (L"WSAIoctl(SIO_ASSOCIATE_PORT_RESERVATION) failed with error = %d\n",
        } else {
            wprintf(L"WSAIoctl(SIO_ASSOCIATE_PORT_RESERVATION) succeeded, bytesReturned = %u\n", 

            service.sin_family = AF_INET;
            service.sin_addr.s_addr = INADDR_ANY;
            service.sin_port = 0;

            iResult = bind(sock, (SOCKADDR*) &service, sizeof(service) );
            if (iResult == SOCKET_ERROR)
                wprintf(L"bind failed with error = %d\n", WSAGetLastError());
            else {
                wprintf(L"bind succeeded\n");
                iResult = getsockname(sock, (SOCKADDR*) &sockName, &nameLen);
                if (iResult == SOCKET_ERROR) 
                    wprintf(L"getsockname failed with error = %d\n", WSAGetLastError() );
                else {
                    wprintf(L"getsockname succeeded\n");
                    wprintf(L"Port number allocated = %u\n", ntohs(sockName.sin_port) );

        if (sock != INVALID_SOCKET) {
            iResult = closesocket(sock);
            if (iResult == SOCKET_ERROR) {
                wprintf(L"closesocket failed with error = %d\n", WSAGetLastError());

    return 0;


Minimum supported client Windows Vista [desktop apps only]
Minimum supported server Windows Server 2008 [desktop apps only]
Target Platform Windows
Header iphlpapi.h
Library Iphlpapi.lib
DLL Iphlpapi.dll

See also