inet -- Internet protocol family


#include  <sys/types.h>

#include <netinet/in.h>


The Internet protocol family is a set of protocols using the Internet Protocol (IP) network layer and the Internet address format. The Internet family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IP protocol.


Internet addresses are four-byte quantities, stored in network standard format. The include file <sys/in.h> defines this address as a discriminated union.

Sockets bound to 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];
When using sockets, the sin_family field is specified in host order, and the sin_port and sin_addr fields are specified in network order.

Sockets may be created with the local address INADDR_ANY to affect ``wildcard'' matching on incoming messages. The address in a connect(SSC) or sendto (see send(SSC)) call may be given as INADDR_ANY to mean this ``host''. The distinguished address INADDR_BROADCAST is allowed as a shorthand for the broadcast address on the primary network if the first network configured supports broadcast.

When using the Transport Layer Interface (TLI), transport providers such as tcp(ADMP) support addresses whose length varies from eight to sixteen bytes. The eight byte form is the same as a sockaddr_in without the sin_zero field. The sixteen byte form is identical to sockaddr_in. Additionally, when using TLI, the sin_family field is accepted in either host or network order. For communicating with other implementations via RFS, the preferred form is eight bytes with sin_family in network order.


The Internet protocol family is comprised of the Internet Protocol (IP), Internet Control Message Protocol (ICMP), Internet Group Management Protocol (IGMP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). TCP is used to support the SOCK_STREAM abstraction; UDP is used to support the SOCK_DGRAM abstraction. A raw interface to IP is available by creating an Internet socket of type SOCK_RAW. The ICMP message protocol is accessible from a raw socket.

The 32-bit Internet address contains both network and host parts. It is frequency-encoded; the most-significant bit is clear in Class A addresses, in which the high-order 8 bits are the network number. Class B addresses use the high-order 16 bits as the network field, and Class C addresses have a 24-bit network part. Sites with a cluster of local networks and a connection to the Internet 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 and host 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(S) commands on a datagram socket in the Internet ``communications domain''; they have the same form as the SIOCIFADDR command (see Intro(ADMP)).

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.


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

Intro(ADMP), Intro(S), icmp(ADMP), igmp(ADMP), ioctl(S), ip(ADMP), socket(SSC), tcp(ADMP), udp(ADMP)
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003