inet(7) inet(7)
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
via the socket interface, where they support the SOCKSTREAM,
SOCKDGRAM, and SOCKRAW socket types, or the Transport Level Inter-
face (TLI), where they support the connectionless (TCLTS) and connec-
tion oriented (TCOTSORD) service types.
PROTOCOLS
The Internet protocol family comprises the Internet Protocol (IP), the
Address Resolution Protocol (ARP), the Internet Control Message Proto-
col (ICMP), the Transmission Control Protocol (TCP), and the User
Datagram Protocol (UDP).
IP Is the internetwork datagram delivery protocol that is central
to the Internet protocol family; see ip(7).
TCP Supports the socket interface's SOCKSTREAM abstraction and
TLI's TCOTSORD service type; see udp(7).
UDP Supports the SOCKDGRAM socket abstraction and the TLI TCLTS
service type; see tcp(7). A direct interface to IP is available
via both TLI and the socket interface; see ip(7).
ICMP Is used by the kernel to handle and report errors in protocol
processing. It is also accessible to user programs; see icmp(7).
ARP Is used to translate 32-bit IP addresses into 48-bit Ethernet
addresses; see arp(7).
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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 net-
work 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 SIOC-
SIFADDR command [see if(7)].
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(3N)].
Addresses in the Internet protocol family use the following structure:
struct sockaddrin {
short sinfamily;
ushort sinport;
struct inaddr sinaddr;
char sinzero[8];
};
Library routines are provided to manipulate structures of this form;
see inet(3N).
The sinaddr field of the sockaddrin structure specifies a local or
remote IP address. Each network interface has its own unique IP
address. The special value INADDRANY may be used in this field to
effect "wildcard" matching. Given in a bind(3N) 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
sockaddrin structure given in the bind(3N) call must specify an
inaddr value of either INADDRANY or one of the system's valid IP
addresses. Requests to bind any other address will elicit the error
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EADDRNOTAVAI. When a connect(3N) call is made for a socket that has a
wildcard local address, the system sets the sinaddr field of the
socket to the IP address of the network interface that the packets for
that connection are routed via.
The sinport field of the sockaddrin structure specifies a port
number used by TCP or UDP. The local port address specified in a
bind(3N) call is restricted to be greater than IPPORTRESERVED
(defined in <netinet/in.h>) unless the creating process is running as
the superuser, providing a space of protected port numbers. In addi-
tion, 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 IPPORTRESERVED. A unique local port address is
also picked when a socket which is not bound is used in a connect(3N)
or sendto [see send(3N)] call. This allows programs which do not care
which local port number is used to set up TCP connections by simply
calling socket(3N) and then connect(3N), and to send UDP datagrams
with a socket(3N) call followed by a sendto(3N) 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 SOREUSEADDR socket
option with setsockopt [see getsockopt(3N)].
TLI applies somewhat different semantics to the binding of local port
numbers. These semantics apply when Internet family protocols are used
via the TLI.
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 services exported.
SEE ALSO
ioctl(2), bind(3N), byteorder(3N), connect(3N), getsockopt(3N),
gethostent(3N), getnetent(3N), getprotoent(3N), getservent(3N),
send(3N), socket(3N), arp(7), icmp(7), if(7), ip(7), tcp(7), udp(7).
Network Information Center, DDN Protocol Handbook (3 vols.), Network
Information Center, SRI International, Menlo Park, Calif., 1985.
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