• Linux C includes directives to convert host names (e.g. "www.facebook.com") to IP address (e.g. "31.13.75.17") and vice versa. Functions called gethostbyname and gethostbyaddr, while technically deprecated, are still so prevalent that you should know how to use them. In fact, your B&O textbook only mentions these deprecated functions:

struct hostent *gethostbyname(const char *name);
struct hostent *gethostbyaddr(const char *addr, int len, int type);

• Each function populates a statically allocated struct hostent describing some host machine on the Internet.
  • gethostbyname assumes argument is a host name, as with www.google.com.
  • gethostbyaddr assumes the first argument is a binary representation of an IP address (e.g. not the string "171.64.64.137", but the base address of a character array with ASCII values of 171, 64, 64, and 137 laid down side by side in network byte order. The second argument is usually 4 for IPv4 addresses (the ones we're familiar with), but could be more for IPv6 addresses.
    The third argument is generally AF_INET (for IPv4 addresses), but might be AF_INET6 (for IPv6 addresses) or specify some other address family. We'll rely exclusively on AF_INET in CS110.

• The struct hostent record packages all of the information about a particular host contributing to the Internet.

struct hostent {
  char *h_name;        // official name of host
  char **h_aliases;    // NULL-terminated list of aliases
  int h_addrtype;      // host address type, e.g. AF_INET
  int h_length;        // length of address (4 for IPv4 addresses)
  char **h_addr_list;  // NULL-terminated list of IP addresses
}; // h_addr_list is really a struct in_addr ** when known to be IPv4 addresses

struct in_addr {
  unsigned int s_addr  // stored in network byte order (big endian)
};

• This shouldn't surprise you, as users prefer the host naming scheme behind "www.facebook.com", but network communication ultimately works with binary representation of "31.13.75.17".
• Here's the core of the full program that queries the user for hostnames and uses gethostbyname to surface information about them:

static void publishIPAddressInfo(const string& host) {
  struct hostent *he = gethostbyname(host.c_str());
  if (he == NULL) { // NULL return value means resolution attempt failed
    cout << host << " could not be resolved to an address." << endl;
    return;
  }

  cout << "Official name is \"" << he->h_name << "\"" << endl;
  cout << "IP Addresses: " << endl;
  struct in_addr **addressList = (struct in_addr **) he->h_addr_list;
  while (*addressList != NULL) {
    cout << "+ " << inet_ntoa(**addressList) << endl;
    addressList++;
  }
}    

• Note two implementation features:
  • h_addr_list is typed to be a char * array and implies it's an array of C strings, even dotted quad IP addresses.
    • That's not correct. h_addr_list is really an array of struct in_addr *s.
    • Each in_addr is a gratutitous struct wrapper around an unsigned int, which is just big enough to store the four bytes of an IP address. These four bytes are stored in network byte order (e.g. big endian order)
  • The inet_ntoa accepts struct in_addrs and returns their dotted quad equivalents (e.g. "171.45.34.199") as statically allocated C strings.
• Technically, gethostbyname and gethostbyaddr are deprecated and should replaced by getaddrinfo, getnameinfo, and freeaddrinfo. But the book relies on the deprecated versions, and I still see developers using them, so I'm comfortable using them, too.