inet - Internet protocol family


#include <sys/types.h>

#include <netinet/in.h>


The Internet protocol family implements a collection of protocols 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.


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(4P) and udp(4P). A direct
interface to IP is available using both TLI and the socket interface (see
ip(4P)). ICMP is used by the kernel to handle and report errors in
protocol processing. It is also accessible to user programs (see
icmp(4P)). ARP is used to translate 32-bit IP addresses into 48-bit
Ethernet addresses. See arp(4P).

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.

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.

Get interface network mask.


IP addresses are four byte quantities, stored in network byte order. IP
addresses should be manipulated using the byte order conversion routines.
See byteorder(3C).

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

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 process 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 connections 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 restriction by setting the SO_REUSEADDR socket option with
setsockopt (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.


ioctl(2), byteorder(3C), gethostbyname(3NSL), bind(3SOCKET),
connect(3SOCKET), getnetbyname(3SOCKET), getprotobyname(3SOCKET),
getservbyname(3SOCKET), getsockopt(3SOCKET), send(3SOCKET),
sockaddr(3SOCKET), socket(3SOCKET), arp(4P), icmp(4P), ip(4P), tcp(4P),

Network Information Center, DDN Protocol Handbook (3 vols.), Network
Information Center, SRI International, Menlo Park, Calif., 1985.


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.

August 3, 2000 INET(4P)