Domain Addressing

Legal combinations of protocols and drivers are specified when the kernel is configured.

For example, sockets that share common communication properties, such as naming conventions and protocol address formats, are grouped into address families.

The Linux file /usr/include/bits/socket.h lists all supported address families.

#define AF_UNSPEC    0    // unspecified 
#define AF_UNIX      1    // local to host (pipes, portals) 
#define AF_INET      2    // internetwork: UDP, TCP, etc. 
#define AF_IMPLINK   3    // arpanet imp addresses 
#define AF_CCITT     10   // CCITT protocols, X.25 etc 
#define AF_SNA       11   // IBM SNA 
#define AF_DECnet    12   // DECnet 
#define AF_APPLETALK 16   // Apple Talk 

#define AF_NIT       17   // Network Interface Tap 
#define AF_802       18   // IEEE 802.2, also ISO 8802 
#define AF_OSI       19   // umbrella for all families used by OSI 
#define AF_X25       20   // CCITT X.25 in particular 
#define AF_GOSIP     22   // U.S. Government OSI 
 .....

Processes communicate using the client-server paradigm.

A server process listens to a socket, one end of a bidirectional communication path and the client processes communicate with the server over another socket, the other end of the communication path.

The kernel maintains internal connections and routes data from client to server.