---
name: "inet"
section: "4"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/inet/4"
---

# inet(4)

Internet protocol family

## NAME

**inet** – Internet protocol family

## SYNOPSIS

**#include \<sys/types.h\>**

**#include \<netinet/in.h\>**

## DESCRIPTION

The Internet protocol family is a collection of protocols layered atop the *Internet* *Protocol* (IP) transport layer, and utilizing 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.

## ADDRESSING

Internet addresses are four byte quantities, stored in network standard format (on the VAX these are word and byte reversed).  The include file ⟨*netinet/in.h*⟩ defines this address as a discriminated union.

Sockets bound to the Internet protocol family utilize the following addressing structure,

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

Sockets may be created with the local address INADDR\_ANY to effect “wildcard” matching on incoming messages.  The address in a [connect(2)](/connect/2.md), [connectx(2)](/connectx/2.md) or [sendto(2)](/sendto/2.md) 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.

## PROTOCOLS

The Internet protocol family is comprised of the IP transport protocol, Internet Control Message Protocol (ICMP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). TCP is used to support the SOCK\_STREAM abstraction while 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(2)](/ioctl/2.md) commands on a datagram socket in the Internet domain; they have the same form as the SIOCIFADDR command (see [intro(4)](/intro/4.md)).

- SIOCSIFNETMASK

  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.

- SIOCGIFNETMASK

  Get interface network mask.

## SEE ALSO

[ioctl(2)](/ioctl/2.md), [socket(2)](/socket/2.md), [icmp(4)](/icmp/4.md), [intro(4)](/intro/4.md), [ip(4)](/ip/4.md), [tcp(4)](/tcp/4.md), [udp(4)](/udp/4.md) “An Introductory 4.3 BSD Interprocess Communication Tutorial”, *PS1*, 7. “An Advanced 4.3 BSD Interprocess Communication Tutorial”, *PS1*, 8.

## CAVEAT

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.

## HISTORY

The **inet** protocol interface appeared in 4.2BSD.