What is AF_INET, and why do I need it?
AF_INET is an address family that is used to designate the type of addresses that your socket can communicate with (in this case, Internet Protocol v4 addresses). When you create a socket, you have to specify its address family, and then you can only use addresses of that type with the socket. The Linux kernel, for example, supports 29 other address families such as UNIX (AF_UNIX) sockets and IPX (AF_IPX), and also communications with IRDA and Bluetooth (AF_IRDA and AF_BLUETOOTH, but it is doubtful you'll use these at such a low level).
For the most part, sticking with AF_INET for socket programming over a network is the safest option. There is also AF_INET6 for Internet Protocol v6 addresses.
Hope this helps,
The primary purpose of AF_INET was to allow for other possible network protocols or address families (AF is for address family; PF_INET is for the (IPv4) internet protocol family). For example, there probably are a few Netware SPX/IPX networks around still; there were other network systems like DECNet, StarLAN and SNA, not to mention the ill-begotten ISO OSI (Open Systems Interconnection), and these did not necessarily use the now ubiquitous IP address to identify the peer host in network connections.
The ubiquitous alternative to AF_INET (which, in retrospect, should have been named AF_INET4) is AF_INET6, for the IPv6 address family. IPv4 uses 32-bit addresses; IPv6 uses 128-bit addresses.
You may see some other values - but they are unusual. It is there to allow for alternatives and future directions. The sockets interface is actually very general indeed - which is one of the reasons it has thrived where other networking interfaces have withered.
Life has (mostly) gotten simpler - be grateful.
Socket are characterized by their domain, type and transport protocol. Common domains are:
AF_UNIX: address format is UNIX pathname
AF_INET: address format is host and port number
(there are actually many other options which can be used here for specialized purposes).usually we use AF_INET for socket programming
Reference: http://www.cs.uic.edu/~troy/fall99/eecs471/sockets.html