AF_INET address family

This address family provides interprocess communication between processes that run on the same system or on different systems.

Addresses for AF_INET sockets are IP addresses and port numbers. You can specify an IP address for an AF_INET socket either as an IP address (such as or in its 32–bit form (X'82638001').

For a socket application that uses the Internet Protocol version 4 (IPv4), the AF_INET address family uses the sockaddr_in address structure. When you use _XOPEN_SOURCE macro, the AF_INET address structure changes to be compatible with BSD 4.4/ UNIX 98 specifications. For the sockaddr_in address structure, these differences are summarized in the table:

Table 1. Differences between BSD 4.3 and BSD 4.4/ UNIX 98 for sockaddr_in address structure
BSD 4.3 sockaddr_in address structure BSD 4.4/ UNIX 98 sockaddr_in address structure
struct sockaddr_in {
  short          sin_family;
  u_short        sin_port;
  struct in_addr sin_addr;
  char           sin_zero[8];
struct sockaddr_in {
  uint8_t         sin_len;
  sa_family_t     sin_family;
  u_short         sin_port;
  struct in_addr  sin_addr;
  char            sin_zero[8];
Table 2. AF_INET address structure
Address structure field Definition
sin_len This field contains the length of the address for UNIX 98 specifications.
Note: The sin_len field is provided only for BSD 4.4 compatibility. It is not necessary to use this field even for BSD 4.4/ UNIX 98 compatibility. The field is ignored on input addresses.
sin_family This field contains the address family, which is always AF_INET when TCP or User Datagram Protocol (UDP) is used.
sin_port This field contains the port number.
sin_addr This field contains the IP address.
sin_zero This field is reserved. Set this field to hexadecimal zeros.