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man pages section 3: Basic Library Functions

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Updated: Thursday, June 13, 2019
 
 

inet(3C)

Name

inet, inet6, inet_ntop, inet_pton, inet_aton, inet_addr, inet_network, inet_makeaddr, inet_lnaof, inet_netof, inet_ntoa - Internet address manipulation

Synopsis

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

const char *inet_ntop(int af, const void *addr, char *cp,
     socklen_t size);
int inet_pton(int af, const char *cp, void *addr);
int inet_aton(const char *cp, struct in_addr *addr);
in_addr_t inet_addr(const char *cp);
in_addr_t inet_network(const char *cp);
struct in_addr inet_makeaddr(in_addr_t net, in_addr_t lna);
in_addr_t inet_lnaof(struct in_addr in);
in_addr_t inet_netof(struct in_addr in);
char *inet_ntoa(struct in_addr in);

Description

The inet_ntop() and inet_pton() functions can manipulate both IPv4 and IPv6 addresses. The inet_aton(), inet_addr(), inet_network(), inet_makeaddr(), inet_lnaof(), inet_netof(), and inet_ntoa() functions can only manipulate IPv4 addresses.

The inet_ntop() function converts a numeric address into a string suitable for presentation. The af argument specifies the family of the address which can be AF_INET or AF_INET6. The addr argument points to a buffer that holds an IPv4 address if the af argument is AF_INET. The addr argument points to a buffer that holds an IPv6 address if the af argument is AF_INET6. The address must be in network byte order. The cp argument points to a buffer where the function stores the resulting string. The application must specify a non-NULL cp argument. The size argument specifies the size of this buffer. For IPv6 addresses, the buffer must be at least 46-octets. For IPv4 addresses, the buffer must be at least 16-octets. To allow applications to easily declare buffers of the proper size to store IPv4 and IPv6 addresses in string form, the following two constants are defined in <netinet/in.h>:

#define INET_ADDRSTRLEN    16
#define INET6_ADDRSTRLEN   46

The inet_pton() function converts the standard text presentation form of an address into its numeric binary form. The af argument specifies the family of the address. Currently, the AF_INET and AF_INET6 address families are supported. The cp argument points to the string being passed in. The addr argument points to a buffer where the function stores the numeric address. The calling application must ensure that the buffer referred to by addr is large enough to hold the numeric address, at least 4 bytes for AF_INET or 16 bytes for AF_INET6.

The inet_aton(), inet_addr(), and inet_network() functions interpret character strings that represent numbers expressed in the IPv4 standard '.' notation, returning numbers suitable for use as IPv4 addresses and IPv4 network numbers, respectively. The inet_makeaddr() function uses an IPv4 network number and a local network address to construct an IPv4 address. The inet_netof() and inet_lnaof() functions break apart IPv4 host addresses, then return the network number and local network address, respectively.

The inet_ntoa() function returns a pointer to a string in the base 256 notation d.d.d.d. See the following section on IPv4 addresses.

Internet addresses are returned in network order, bytes ordered from left to right. Network numbers and local address parts are returned as machine format integer values.

IPv6 Addresses

    There are three conventional forms for representing IPv6 addresses as strings:

  1. The preferred form is x:x:x:x:x:x:x:x, where the 'x's are the hexadecimal values of the eight 16-bit pieces of the address. For example:

    1080:0:0:0:8:800:200C:417A

    It is not necessary to write the leading zeros in an individual field. There must be at least one numeral in every field, except when the special syntax described in the following is used.

  2. It is common for addresses to contain long strings of zero bits in some methods used to allocate certain IPv6 address styles. A special syntax is available to compress the zeros. The use of “::” indicates multiple groups of 16 bits of zeros. The :: may only appear once in an address. The :: can also be used to compress the leading and trailing zeros in an address. For example:

    1080::8:800:200C:417A
  3. The alternative form x:x:x:x:x:x:d.d.d.d is sometimes more convenient when dealing with a mixed environment of IPv4 and IPv6 nodes. The x's in this form represent the hexadecimal values of the six high-order 16-bit pieces of the address. The d's represent the decimal values of the four low-order 8-bit pieces of the standard IPv4 address. For example:

    ::FFFF:129.144.52.38
    ::129.144.52.38
    
    

    The ::FFFF:d.d.d.d and ::d.d.d.d pieces are the general forms of an IPv4–mapped IPv6 address and an IPv4–compatible IPv6 address.

    The IPv4 portion must be in the d.d.d.d form. The following forms are invalid:

    ::FFFF:d.d.d
    ::FFFF:d.d
    ::d.d.d
    ::d.d

    The ::FFFF:d form is a valid but unconventional representation of the IPv4–compatible IPv6 address ::255.255.0.d.

    The ::d form corresponds to the general IPv6 address 0:0:0:0:0:0:0:d.

IPv4 Addresses

Values specified using `.' notation take one of the following forms:

d.d.d.d
d.d.d
d.d
d

When four parts are specified, each part is interpreted as a byte of data and assigned from left to right to the four bytes of an IPv4 address.

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. The three part address format is convenient for specifying Class B network addresses such 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. The two part address format is convenient for specifying Class A network addresses such as net.host.

When only one part is given, the value is stored directly in the network address without any byte rearrangement.

With the exception of inet_pton(), numbers supplied as parts in '.' notation may be decimal, octal, or hexadecimal, as specified in C language. For example, a leading 0x or 0X implies hexadecimal. A leading 0 implies octal. Otherwise, the number is interpreted as decimal.

For IPv4 addresses, inet_pton() accepts only a string in standard IPv4 dot notation:

d.d.d.d

Each number has one to three digits with a decimal value between 0 and 255.

The inet_addr() function has been obsoleted by inet_aton().

Return Values

The inet_aton() function returns nonzero if the address is valid, 0 if the address is invalid.

The inet_ntop() function returns a pointer to the buffer that contains a string if the conversion succeeds. Otherwise, NULL is returned. Upon failure, errno is set to EAFNOSUPPORT if the af argument is invalid or ENOSPC if the size of the result buffer is inadequate.

The inet_pton() function returns 1 if the conversion succeeds, 0 if the input is not a valid IPv4 dotted-decimal string or a valid IPv6 address string. The function returns –1 with errno set to EAFNOSUPPORT if the af argument is unknown.

The value INADDR_NONE, which is equivalent to (in_addr_t)(-1), is returned by inet_addr() and inet_network() for malformed requests.

The functions inet_netof() and inet_lnaof() break apart IPv4 host addresses, returning the network number and local network address part, respectively.

The function inet_ntoa() returns a pointer to a string in the base 256 notation d.d.d.d, described in the section on IPv4 addresses.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
See below.
MT-Level
Safe

The inet_ntop(), inet_pton(), inet_aton(), inet_addr(), and inet_network() functions are Committed. The inet_lnaof(), inet_makeaddr(), inet_netof(), and inet_network() functions are Committed (Obsolete).

See Also

gethostbyname(3C), getipnodebyname(3C), getnetbyname(3C), inet.h(3HEAD), hosts(5), networks(5), attributes(7)

Notes

The return value from inet_ntoa() points to a buffer which is overwritten on each call. This buffer is implemented as thread-specific data in multithreaded applications.

IPv4-mapped addresses are not recommended.

Bugs

The problem of host byte ordering versus network byte ordering is confusing. A simple way to specify Class C network addresses in a manner similar to that for Class B and Class A is needed.