There are special socket functions for converting host names to network addresses and vice versa in the AF_INET and AF_INET6 address families.
Socket functions for converting addresses in the AF_INET and AF_INET6 address families
The getipnodebyname() function converts a host name to an IPv4 or IPv6 address. A host name is passed when getipnodebyname() is called.
The getipnodebyaddr() function converts an IPv4 or IPv6 address to a host name. An IPv4 or IPv6 address is passed when getipnodebyaddr() is called.
The inet_ntop() function converts an Internet host name to a character string. This character string is returned as follows:
in hexadecimal colon notation for AF_INET6
in decimal dotted notation for AF_INET
The inet_pton() function converts an Internet host address in printable representation
from a character string in decimal dotted notation to a binary IPv4 address (AF_INET).
from a character string in hexadecimal colon notation to a binary IPv6 address (AF_INET6).
Abbreviated notation using two consecutive colons “::” is not supported for AF_INET6.
Socket functions address conversion which are only supported in AF_INET
The gethostbyname() function converts a host name to an IPv4 address. A host name is passed when gethostbyname() is called.
The gethostbyaddr() function converts an IPv4 address to a host name. An IPv4 address is passed when gethostbyaddr() is called.
gethostbyname() and gethostbyaddr(), as well as getipnodebyname() and getipnodebyaddr(), return a pointer to an object of data type struct hostent as their result.
The hostent structure is declared in <netdb.h> as follows:
struct hostent {
char *h_name; /* official host name */
char **h_aliases; /* alias list */
int h_addrtype; /* address type */
int h_length; /* length of the address (in bytes) */
char **h_addr_list; /* list of addresses for the host, */
/* terminated with the null pointer*/
};
#define h_addr h_addr_list[0] /* first address, network byte order */
The hostent object returned by gethostbyname() and gethostbyaddr() and by getipnodebyname() and getipnodebyaddr() always contains the following information, if this is made available by the database:
the official name of the host
a list of the host aliases
address type (domain)
a list of addresses of variable length, terminated with the null pointer
The address list is required because a host normally has several addresses which are all assigned to the same host name. h_addr ensures backward compatibility and is defined as the first address in the address list of the hostent structure.
The inet_ntoa() function converts an IPv4 host address to a character string in accordance with the normal Internet dotted notation.