INET(4)

NAME
inet - Internet protocol family

SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>

DESCRIPTION
The Internet protocol family is a collection of protocols layered atop the Internet Protocol (IP) transport layer, and utilizing the Internet address format. The Internet family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IP protocol.

ADDRESSING
Internet addresses are four byte quantities, stored in network standard format. The include file <netinet/in.h> defines this address as a discriminated union.

Sockets bound to the Internet protocol family utilize the following addressing structure,

    struct sockaddr_in {
    	u_int8_t	sin_len;
    	sa_family_t	sin_family;
    	in_port_t	sin_port;
    	struct in_addr	sin_addr;
    	int8_t		sin_zero[8];
    };

Sockets may be created with the local address INADDR_ANY to effect “wildcard” matching on incoming messages. The address in a connect(2) or sendto(2) call may be given as INADDR_ANY to mean “this host”. The distinguished address INADDR_BROADCAST is allowed as a shorthand for the broadcast address on the primary network if the first network configured supports broadcast.

PROTOCOLS
The Internet protocol family is comprised of the IP transport protocol, Internet Control Message Protocol (ICMP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). TCP is used to support the SOCK_STREAM abstraction while UDP is used to support the SOCK_DGRAM abstraction. A raw interface to IP is available by creating an Internet socket of type SOCK_RAW. The ICMP message protocol is accessible from a raw socket.

The 32-bit Internet address contains both network and host parts. It is frequency-encoded; the most-significant bit is clear in Class A addresses, in which the high-order 8 bits are the network number. Class B addresses use the high-order 16 bits as the network field, and Class C addresses have a 24-bit network part. Sites with a cluster of local networks and a connection to the Internet may choose to use a single network number for the cluster; this is done by using subnet addressing. The local (host) portion of the address is further subdivided into subnet and host parts. Within a subnet, each subnet appears to be an individual network; externally, the entire cluster appears to be a single, uniform network requiring only a single routing entry. Subnet addressing is enabled and examined by the following ioctl(2) commands on a datagram socket in the Internet domain; they have the same form as the SIOCIFADDR command (see netintro(4)).

SIOCSIFNETMASK
    Set interface network mask. The network mask defines the network part of the address; if it contains more of the address than the address type would indicate, then subnets are in use.

SIOCGIFNETMASK
    Get interface network mask.

SEE ALSO
ioctl(2), socket(2), inet_net_ntop(3), inet_ntop(3), icmp(4), ip(4), netintro(4), tcp(4), udp(4) An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7. An Advanced 4.3 BSD Interprocess Communication Tutorial, PS1, 8.

HISTORY
The inet protocol interface appeared in 4.2BSD.

CAVEATS
The Internet protocol support is subject to change as the Internet protocols develop. Users should not depend on details of the current implementation, but rather the services exported.