--- name: "inet" section: "3" platform: "macos" package: "Libc" version: "1752.120.2" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/inet" --- # inet(3) Internet address manipulation routines ## NAME **inet\_addr**, **inet\_aton**, **inet\_lnaof**, **inet\_makeaddr**, **inet\_netof**, **inet\_network**, **inet\_ntoa**, **inet\_ntop**, **inet\_pton** – Internet address manipulation routines ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *in\_addr\_t*\ **inet\_addr**(*const char \*cp*); *int*\ **inet\_aton**(*const char \*cp*, *struct in\_addr \*pin*); *in\_addr\_t*\ **inet\_lnaof**(*struct in\_addr in*); *struct* *in\_addr*\ **inet\_makeaddr**(*in\_addr\_t net*, *in\_addr\_t lna*); *in\_addr\_t*\ **inet\_netof**(*struct in\_addr in*); *in\_addr\_t*\ **inet\_network**(*const char \*cp*); *char* *\**\ **inet\_ntoa**(*struct in\_addr in*); *const* *char* *\**\ **inet\_ntop**(*int af*, *const void \* restrict src*, *char \* restrict dst*, *socklen\_t size*); *int*\ **inet\_pton**(*int af*, *const char \* restrict src*, *void \* restrict dst*); ## DESCRIPTION The routines **inet\_aton**(), **inet\_addr**() and **inet\_network**() interpret character strings representing numbers expressed in the Internet standard ‘.’ notation. The **inet\_pton**() function converts a presentation format address (that is, printable form as held in a character string) to network format (usually a *struct* *in\_addr* or some other internal binary representation, in network byte order). It returns 1 if the address was valid for the specified address family, or 0 if the address was not parseable in the specified address family, or -1 if some system error occurred (in which case *errno* will have been set). This function is presently valid for AF\_INET and AF\_INET6. The **inet\_aton**() routine interprets the specified character string as an Internet address, placing the address into the structure provided. It returns 1 if the string was successfully interpreted, or 0 if the string is invalid. The **inet\_addr**() and **inet\_network**() functions return numbers suitable for use as Internet addresses and Internet network numbers, respectively. The function **inet\_ntop**() converts an address *\*src* from network format (usually a *struct* *in\_addr* or some other binary form, in network byte order) to presentation format (suitable for external display purposes). The *size* argument specifies the size, in bytes, of the buffer *\*dst*. INET\_ADDRSTRLEN and INET6\_ADDRSTRLEN define the maximum size required to convert an address of the respective type. It returns NULL if a system error occurs (in which case, *errno* will have been set), or it returns a pointer to the destination string. This function is presently valid for AF\_INET and AF\_INET6. The routine **inet\_ntoa**() takes an Internet address and returns an ASCII string representing the address in ‘.’ notation. The routine **inet\_makeaddr**() takes an Internet network number and a local network address and constructs an Internet address from it. The routines **inet\_netof**() and **inet\_lnaof**() break apart Internet host addresses, returning the network number and local network address part, respectively. All Internet addresses are returned in network order (bytes ordered from left to right). All network numbers and local address parts are returned as machine byte order integer values. ## INTERNET ADDRESSES Values specified using the ‘.’ notation take one of the following forms: ``` a.b.c.d a.b.c a.b a ``` When four parts are specified, each is interpreted as a byte of data and assigned, from left to right, to the four bytes of an Internet address. Note that when an Internet address is viewed as a 32-bit integer quantity on the VAX the bytes referred to above appear as “d.c.b.a”. That is, VAX bytes are ordered from right to left. When a three part address is specified, the last part is interpreted as a 16-bit quantity and placed in the right-most two bytes of the network address. This makes the three part address format convenient for specifying Class B network addresses as “128.net.host”. When a two part address is supplied, the last part is interpreted as a 24-bit quantity and placed in the right most three bytes of the network address. This makes the two part address format convenient for specifying Class A network addresses as “net.host”. When only one part is given, the value is stored directly in the network address without any byte rearrangement. All numbers supplied as “parts” in a ‘.’ notation may be decimal, octal, or hexadecimal, as specified in the C language (i.e., a leading 0x or 0X implies hexadecimal; otherwise, a leading 0 implies octal; otherwise, the number is interpreted as decimal). ## DIAGNOSTICS The constant INADDR\_NONE is returned by **inet\_addr**() and **inet\_network**() for malformed requests. ## ERRORS The **inet\_ntop**() call fails if: - \[EAFNOSUPPORT\] *\*src* was not an AF\_INET or AF\_INET6 family address. - \[ENOSPC\] *size* was not large enough to store the presentation form of the address. ## LEGACY SYNOPSIS **#include \** **#include \** **#include \** **#include \** These include files are necessary for all functions. ## SEE ALSO [byteorder(3)](/byteorder/3.md), [getaddrinfo(3)](/getaddrinfo/3.md), [gethostbyname(3)](/gethostbyname/3.md), [getnameinfo(3)](/getnameinfo/3.md), [getnetent(3)](/getnetent/3.md), [inet\_net(3)](/inet_net/3.md), [hosts(5)](/hosts/5.md), [networks(5)](/networks/5.md) *IP Version 6 Addressing Architecture*, RFC, 2373, July 1998. ## STANDARDS The **inet\_ntop**() and **inet\_pton**() functions conform to X/Open Networking Services Issue 5.2 (“XNS5.2”). Note that **inet\_pton**() does not accept 1-, 2-, or 3-part dotted addresses; all four parts must be specified and are interpreted only as decimal values. Any leading 0s will be stripped. This is a narrower input set than that accepted by **inet\_aton**(). ## HISTORY These functions appeared in 4.2BSD. ## BUGS The value INADDR\_NONE (0xffffffff) is a valid broadcast address, but **inet\_addr**() cannot return that value without indicating failure. The newer **inet\_aton**() function does not share this problem. The problem of host byte ordering versus network byte ordering is confusing. The string returned by **inet\_ntoa**() resides in a static memory area. The **inet\_addr**() function should return a *struct*, *in\_addr*.