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Manual page for INET(4F)

inet - Internet protocol family


options INET

#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 provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket 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 is used to support the SOCK_STREAM abstraction while UDP is used to support the SOCK_DGRAM abstraction; see tcp.4p and udp.4p A raw interface to IP is available by creating an Internet socket of type SOCK_RAW; 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 are 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 local networks may chose 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 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 on a datagram socket in the Internet domain; they have the same form as the SIOCIFADDR command (see intro.4

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 (on Sun386i systems these are word and byte reversed).

Sockets in the Internet protocol family use the following addressing structure:

struct sockaddr_in {
	short	sin_family;
	u_short	sin_port;
	struct	in_addr sin_addr;
	char	sin_zero[8];

Library routines are provided to manipulate structures of this form; see intro.3

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.2 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.2 call must specify an in_addr value of either IPADDR_ANY or one of the system's valid IP addresses. Requests to bind any other address will elicit the error EADDRNOTAVAIL. When a connect.2 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 via.

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.2 call is restricted to be greater than IPPORT_RESERVED (defined in <netinet/in.h>) unless the creating process is running as the super-user, 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.2 or send.2 call. This allows programs which do not care which local port number is used to set up TCP connections by simply calling socket.2 and then connect.2 and to send UDP datagrams with a socket.2 call followed by a send.2 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.2


bind.2 connect.2 getsockopt.2 ioctl.2 send.2 socket.2 intro.3 byteorder.3n gethostent.3n getnetent.3n getprotoent.3n getservent.3n inet.3n intro.4 arp.4p icmp.4p ip.4p tcp.4p udp.4p

Network Information Center, DDN Protocol Handbook (3 vols.), Network Information Center, SRI International, Menlo Park, Calif., 1985.
A 4.2BSD Interprocess Communication Primer


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.

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Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).

Last modified 21/April/97