INET_NTOP(3) | Library Functions Manual | INET_NTOP(3)

# NAME

`inet_ntop`, `inet_pton` — convert Internet addresses between presentation and network formats

# SYNOPSIS

`#include <sys/socket.h>`
`#include <arpa/inet.h>`

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 `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; 0 if the address wasn't 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 function `inet_ntop`() converts an address from network format to presentation format. 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.

All Internet addresses are returned in network order (bytes ordered from left to right).

# INTERNET ADDRESSES (IP VERSION 4)

Values must be specified using the standard dot notation:

    a.b.c.d

All four parts must be decimal numbers between 0 and 255, inclusive, and are 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 a system that uses little-endian byte order (such as AMD64 or ARM processors) the bytes referred to above appear as “`d.c.b.a`”. That is, little-endian bytes are ordered from right to left.

# INTERNET ADDRESSES (IP VERSION 6)

In order to support scoped IPv6 addresses, getaddrinfo(3) and getnameinfo(3) are recommended rather than the functions presented here.

The presentation format of an IPv6 address is given in RFC 4291:

There are three conventional forms for representing IPv6 addresses as text 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. Examples:

       FEDC:BA98:7654:3210:FEDC:BA98:7654:3210
       1080:0:0:0:8:800:200C:417A

   Note that it is not necessary to write the leading zeros in an individual field, but there must be at least one numeral in every field (except for the case described in 2.).

2. Due to the method of allocating certain styles of IPv6 addresses, it will be common for addresses to contain long strings of zero bits. In order to make writing addresses containing zero bits easier, a special syntax is available to compress the zeros. The use of “::” indicates multiple groups of 16 bits of zeros. The “::” can only appear once in an address. The “::” can also be used to compress the leading and/or trailing zeros in an address.

   For example the following addresses:

       1080:0:0:0:8:800:200C:417A  a unicast address
       FF01:0:0:0:0:0:0:43         a multicast address
       0:0:0:0:0:0:0:1             the loopback address
       0:0:0:0:0:0:0:0             the unspecified addresses

   may be represented as:

       1080::8:800:200C:417A       a unicast address
       FF01::43                    a multicast address
       ::1                         the loopback address
       ::                          the unspecified addresses

3. An alternative form that is sometimes more convenient when dealing with a mixed environment of IPv4 and IPv6 nodes is x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values of the six high-order 16-bit pieces of the address, and the 'd's are the decimal values of the four low-order 8-bit pieces of the address (standard IPv4 representation). Examples:

       0:0:0:0:0:0:13.1.68.3
       0:0:0:0:0:FFFF:129.144.52.38

   or in compressed form:

       ::13.1.68.3
       ::FFFF:129.144.52.38

# SEE ALSO

gethostbyname(3), inet_addr(3), inet_net_ntop(3), hosts(5)

# STANDARDS

The `inet_ntop` and `inet_pton` functions conform to the IETF IPv6 BSD API and address formatting specifications, as well as IEEE Std 1003.1-2008 (“POSIX.1”).

# HISTORY

The `inet_pton` and `inet_ntop` functions appeared in BIND 4.9.4.

# CAVEATS

Note that `inet_pton` does not accept 1-, 2-, or 3-part dotted addresses; all four parts must be specified and must be in decimal (and not octal or hexadecimal). This is a narrower input set than that accepted by `inet_aton`.

R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 3493, February 2003.

R. Hinden and S. Deering, IP Version 6 Addressing Architecture, RFC 4291, February 2006.

September 11, 2022 | Debian