.\"	$OpenBSD: inet.4,v 1.17 2015/09/10 17:55:21 schwarze Exp $
.\"	$NetBSD: inet.4,v 1.3 1994/11/30 16:22:18 jtc Exp $
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.\"     @(#)inet.4	8.1 (Berkeley) 6/5/93
.\"
.Dd $Mdocdate: September 10 2015 $
.Dt INET 4
.Os
.Sh NAME
.Nm inet
.Nd Internet protocol family
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.Sh DESCRIPTION
The Internet protocol family is a collection of protocols
layered atop the
.Em Internet Protocol
.Pq Tn IP
transport layer, and utilizing the Internet address format.
The Internet family provides protocol support for the
.Dv SOCK_STREAM ,
.Dv SOCK_DGRAM ,
and
.Dv SOCK_RAW
socket types; the
.Dv SOCK_RAW
interface provides access to the
.Tn IP
protocol.
.Sh ADDRESSING
Internet addresses are four byte quantities, stored in
network standard format (on the
.Tn VAX
these are word and byte
reversed).
The include file
.In netinet/in.h
defines this address as a discriminated union.
.Pp
Sockets bound to the Internet protocol family utilize
the following addressing structure,
.Bd -literal -offset indent
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];
};
.Ed
.Pp
Sockets may be created with the local address
.Dv INADDR_ANY
to effect
.Dq wildcard
matching on incoming messages.
The address in a
.Xr connect 2
or
.Xr sendto 2
call may be given as
.Dv INADDR_ANY
to mean
.Dq this host .
The distinguished address
.Dv INADDR_BROADCAST
is allowed as a shorthand for the broadcast address on the primary
network if the first network configured supports broadcast.
.Sh PROTOCOLS
The Internet protocol family is comprised of
the
.Tn IP
transport protocol, Internet Control
Message Protocol
.Pq Tn ICMP ,
Transmission Control
Protocol
.Pq Tn TCP ,
and User Datagram Protocol
.Pq Tn UDP .
.Tn TCP
is used to support the
.Dv SOCK_STREAM
abstraction while
.Tn UDP
is used to support the
.Dv SOCK_DGRAM
abstraction.
A raw interface to
.Tn IP
is available
by creating an Internet socket of type
.Dv SOCK_RAW .
The
.Tn ICMP
message protocol is accessible from a raw socket.
.Pp
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
.Xr ioctl 2
commands on a datagram socket in the Internet domain;
they have the same form as the
.Dv SIOCIFADDR
command (see
.Xr netintro 4 ) .
.Bl -tag -width SIOCSIFNETMASK
.It Dv 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.
.It Dv SIOCGIFNETMASK
Get interface network mask.
.El
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr socket 2 ,
.Xr inet_net 3 ,
.Xr inet_ntop 3 ,
.Xr icmp 4 ,
.Xr ip 4 ,
.Xr netintro 4 ,
.Xr tcp 4 ,
.Xr udp 4
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re
.Sh HISTORY
The
.Nm
protocol interface appeared in
.Bx 4.2 .
.Sh 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.