Answer:
Below is the header files and pakage mentioned with respect to language/platform.
Java:
import java.net.*; //pakage.
C#
using System.Net;
using System.Net.Sockets;
using System.Net.NetworkInformation;
C/C++ Windows
#include <winsock2.h>
#include <ws2tcpip.h>
Need to link with Ws2_32.lib
#pragma comment (lib, "Ws2_32.lib")
WinCE
#include <winsock2.h>
#include <ws2tcpip.h>
Need to link with Ws2.lib
#pragma comment(lib,"ws2.lib")
C/C++ Linux
#include <sys/socket.h> // for socket(), bind(), listen(), accept()
#include <netinet/in.h> // for PF_INET, SOCK_STREAM, IPPROTO_TCP
#include // for read(), write(), close()
Note: This Question & answers sound simple but in interview many people forget it. Generally, it is asked to fresher but it is also asked to experience people too.