stdio.h, header file contains declarations used in most input and output and is typically included in all C programs.

sys/types.h, header file contains definitions of a number of data types used in system calls. These types are used in the next two include files.

sys/socket.h, header file socket.h includes a number of definitions of structures needed for sockets.

netinet/in.h, header file in.h contains constants and structures needed for internet domain addresses.