gethostbyname
Prototype: 
#include <netdb.h>
struct hostent *gethostbyname(const char *name);
General Description: Search for and translate host name to an IP address. The name may be a host name or address. If it is an address, the call does no searches, rather it returns the address in the h_name and h_addr_list[0] fields of the hostent structure. If the host name ends with a period, the call treats the name as an absolute name with no abbreviation. Otherwise, the call searches the local subnetwork names. If you have HOSTALIASES defined in your environment, the call searches the file HOSTALIASES points to.
Return Value: The call returns a pointer a struct hostent; if it fails, the return value is NULL. The structure lists all the names and addresses the host owns. The macro h_addr provides backwards compatibility. 
#define h_addr h_addr_list[0]
struct hostent {
	char *h_name;       /* official name of host */
	char **h_aliases;   /* alias list */
	int h_addrtype;     /* host address type */
	int h_length;       /* length of address */
	char **h_addr_list; /* list of addresses; 0th is the primary */
};
Parameters
name The host's name to search -or- the IP address.
Possible Errors
ENOTFOUND The specified host is unknown.NO_ADDRESS or NO_DATAThe requested name is valid but does not have an IP address.
NO_RECOVERY A non-recoverable name server error occurred.
EAGAIN A temporary error occurred on an authoritative name server. Try again later.
Examples 
int i;
struct hostent *host;
host = gethostbyname("sunsite.unc.edu");
if ( host != NULL )
{
    printf("Official name: %s\n", host->h_name);
    for ( i = 0; host->h_aliases[i] != 0; i++ )
        printf("   alias[%d]: %s\n", i+1,
            host->h_aliases[i]);
    printf("Address type=%d\n", host->h_addrtype);
    for ( i = 0; i < host->h_length; i++ )
        printf("Addr[%d]: %s\n", i+1,
            inet_ntoa(host->h_addr_list[i]));
}
else
    perror("sunsite.unc.edu");