Address Families

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The socket subroutine takes an address family as a parameter.

Specifying an address family indicates to the system how to interpret supplied addresses. The /usr/include/sys/socket.h and /usr/include/sys/socketvar.h files define the address families.

A socket subroutine that takes an address family (AF) as a parameter can use AF_UNIX (UNIX), AF_INET (Internet), AF_NS (Xerox Network Systems), or AF_NDD (Network Device Drivers of the operating sytem) protocol. These address families are part of the following communication domains:
Item Description
UNIX Provides socket communication between processes running on the same operating system when an address family of AF_UNIX is specified. A socket name in the UNIX domain is a string of ASCII characters whose maximum length depends on the machine in use.
Internet Provides socket communication between a local process and a process running on a remote host when an address family of AF_INET is specified. The Internet domain requires that Transmission Control Protocol/Internet Protocol (TCP/IP) be installed on your system. A socket name in the Internet domain is an Internet address, made up of a 32-bit IP address and a 16-bit port address.
NDD Provides socket communication between a local process and a process running on a remote host when an address family of AF_NDD is specified. The NDD domain enables applications to run directly on top of physical networks. This is in contrast to the Internet domain, in which applications run on top of transport protocols such as TCP, or User Datagram Protocol (UDP). A socket name in the NDD domain consists of operating system NDD name and a second part that is protocol dependent.

Communication domains are described by a domain data structure that is loadable. Communication protocols within a domain are described by a structure that is defined within the system for each protocol implementation configured. When a request is made to create a socket, the system uses the name of the communication domain to search linearly the list of configured domains. If the domain is found, the domain's table of supported protocols is consulted for a protocol appropriate for the type of socket being created or for a specific protocol request. (A wildcard entry may exist for a raw domain.) Should multiple protocol entries satisfy the request, the first is selected.