inet(7P)
NAME
inet - Internet protocol family
SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>
DESCRIPTION
The Internet protocol family implements a collection of pro-
tocols which are centered around the Internet Protocol
("IP") and which share a common address format. The Internet
family protocols can be accessed using the socket interface,
where they support the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW
socket types, or the Transport Level Interface (TLI), where
they support the connectionless (T_CLTS) and connection
oriented (T_COTS_ORD) service types.
PROTOCOLS
The Internet protocol family is comprised of the Internet
Protocol ("IP"), the Address Resolution Protocol ("ARP"),
the Internet Control Message Protocol ("ICMP"), the
Transmission Control Protocol ("TCP"), and the User Datagram
Protocol ("UDP").
TCP supports the socket interface's SOCK_STREAM abstraction
and TLI's T_COTS_ORD service type. UDP supports the
SOCK_DGRAM socket abstraction and the TLI T_CLTS service
type. See tcp(7P) and udp(7P). A direct interface to IP is
available using both TLI and the socket interface (see
ip(7P)). ICMP is used by the kernel to handle and report
errors in protocol processing. It is also accessible to user
programs (see icmp(7P)). ARP is used to translate 32-bit IP
addresses into 48-bit Ethernet addresses. See arp(7P).
The 32-bit IP address is divided into network number and
host number parts. It is frequency-encoded. The most-
significant bit is zero in Class A addresses, in which the
high-order 8 bits represent the network number. Class B
addresses have their high order two bits set to 10 and use
the high-order 16 bits as the network number field. Class C
addresses have a 24-bit network number part of which the
high order three bits are 110. Sites with a cluster of IP
networks may chose to use a single network number for the
cluster; this is done by using subnet addressing. The host
number portion of the address is further subdivided into
subnet number and host number 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.
They have the same form as the SIOCSIFADDR command.
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.
ADDRESSING
IP addresses are four byte quantities, stored in network
byte order. IP addresses should be manipulated using the
byte order conversion routines. See byteorder(3SOCKET).
Addresses in the Internet protocol family use the
sockaddr_in structure, which has that following members:
short sin_family;
ushort_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
Library routines are provided to manipulate structures of
this form; See inet(3SOCKET).
The sin_addr field of the sockaddr_in structure specifies a
local or remote IP address. Each network interface has its
own unique IP address. The special value INADDR_ANY may be
used in this field to effect "wildcard" matching. Given in a
bind(3SOCKET) call, this value leaves the local IP address
of the socket unspecified, so that the socket will receive
connections or messages directed at any of the valid IP
addresses of the system. This can prove useful when a pro-
cess neither knows nor cares what the local IP address is or
when a process wishes to receive requests using all of its
network interfaces. The sockaddr_in structure given in the
bind(3SOCKET) call must specify an in_addr value of either
INADDR_ANY or one of the system's valid IP addresses.
Requests to bind any other address will elicit the error
EADDRNOTAVAIL. When a connect(3SOCKET) call is made for a
socket that has a wildcard local address, the system sets
the sin_addr field of the socket to the IP address of the
network interface that the packets for that connection are
routed through.
The sin_port field of the sockaddr_in structure specifies a
port number used by TCP or UDP. The local port address
specified in a bind(3SOCKET) call is restricted to be
greater than IPPORT_RESERVED (defined in <<netinet/in.h>>)
unless the creating process is running as the superuser,
providing a space of protected port numbers. In addition,
the local port address must not be in use by any socket of
same address family and type. Requests to bind sockets to
port numbers being used by other sockets return the error
EADDRINUSE. If the local port address is specified as 0,
then the system picks a unique port address greater than
IPPORT_RESERVED. A unique local port address is also picked
when a socket which is not bound is used in a
connect(3SOCKET) or sendto (see send(3SOCKET)) call. This
allows programs which do not care which local port number is
used to set up TCP connections by simply calling
socket(3SOCKET) and then connect(3SOCKET), and to send UDP
datagrams with a socket(3SOCKET) call followed by a sendto()
call.
Although this implementation restricts sockets to unique
local port numbers, TCP allows multiple simultaneous connec-
tions involving the same local port number so long as the
remote IP addresses or port numbers are different for each
connection. Programs may explicitly override the socket res-
triction by setting the SO_REUSEADDR socket option with set-
sockopt (see getsockopt(3SOCKET)).
TLI applies somewhat different semantics to the binding of
local port numbers. These semantics apply when Internet
family protocols are used using the TLI.
SEE ALSO
ioctl(2), bind(3SOCKET), byteorder(3SOCKET),
connect(3SOCKET), gethostbyname(3NSL),
getnetbyname(3SOCKET), getprotobyname(3SOCKET),
getservbyname(3SOCKET), getsockopt(3SOCKET), send(3SOCKET),
socket(3SOCKET), arp(7P), icmp(7P), ip(7P), tcp(7P), udp(7P)
Network Information Center, DDN Protocol Handbook (3 vols.),
Network Information Center, SRI International, Menlo Park,
Calif., 1985.
NOTES
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 ser-
vices exported.
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