Tribblix: manual page: inet.4p

INET(4P) Protocols INET(4P)

NAME

inet - Internet protocol family

SYNOPSIS

#include <sys/types.h>

#include <netinet/in.h>

DESCRIPTION

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.

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(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.

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(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 inet(3C).

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.

NOTES