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Convert IP addresses from a dots-and-number string to a struct in_addr and back
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
char *inet_ntoa(struct in_addr in);
int inet_aton(const char *cp, struct in_addr *inp);
in_addr_t inet_addr(const char *cp);
All of these functions convert from a struct in_addr (part of your struct sockaddr_in, most likely) to a string in dots-and-numbers format (e.g. "192.168.5.10") and vice-versa. If you have an IP address passed on the command line or something, this is the easiest way to get a struct in_addr to connect() to, or whatever. If you need more power, try some of the DNS functions like gethostbyname() or attempt a coup-de-tat in your local country.
The function inet_ntoa() converts a network address in a struct in_addr to a dots-and-numbers format string. The "n" in "ntoa" stands for network, and the "a" stands for ASCII for historical reasons (so it's "Network To ASCII"--the "toa" suffix has an analogous friend in the C library called atoi() which converts an ASCII string to an integer.)
The function inet_aton() is the opposite, converting from a dots-and-numbers string into a in_addr_t (which is the type of the field s_addr in your struct in_addr.)
Finally, the function inet_addr() is an older function that does basically the same thing as inet_aton(). It's theoretically deprecated, but you'll see it alot and the police won't come get you if you use it.
inet_aton() returns non-zero if the address is a valid one, and it returns zero if the address is invalid.
inet_ntoa() returns the dots-and-numbers string in a static buffer that is overwritten with each call to the function.
inet_addr() returns the address as an in_addr_t, or -1 if there's an error. (That is the same result as if you tried to convert the string "255.255.255.255", which is a valid IP address. This is why inet_aton() is better.)
struct sockaddr_in antelope;
char *some_addr;
inet_aton("10.0.0.1", &antelope.sin_addr); // store IP in antelope
some_addr = inet_ntoa(antelope.sin_addr); // return the IP
printf("%s\n", some_addr); // prints "10.0.0.1"
// and this call is the same as the inet_aton() call, above:
antelope.sin_addr.s_addr = inet_addr("10.0.0.1");
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