IP Verifier for Networks class

classful_ip_connectability.c

#include <stdio.h>
#include <stdbool.h>

#define MAX_IP_LEN 15    // Max length is 4*3 + 3, (max of 4 3-digit numbers, plus 3 periods)

typedef unsigned char BYTE;

// Enum for all possible IP classes
typedef enum {
  A, B, C, D, E, LOOPBACK
} IP_CLASS;

// Struct that holds a single IP address (4 bytes + its class)
typedef struct {
  BYTE addr[4];
  IP_CLASS ip_class;
} IP;

// Reads IP from stdin and finds its class
//
// Returns error code
// Negative value for errors, zero for success
int read_ip(IP *input_ip) { 
  printf("Enter IP > ");
  // If > 255 the remainder is assigned
  scanf("%d.%d.%d.%d", &input_ip->addr[0], &input_ip->addr[1], &input_ip->addr[2], &input_ip->addr[3]);

  const BYTE class_byte = input_ip->addr[0];   // Save a copy for less typing
  IP_CLASS ip_class;

  if      (class_byte < 127)  {ip_class = A;}
  else if (class_byte == 127) {ip_class = LOOPBACK; return -1;}  // 127... is reserved (loopback address)
  else if (class_byte < 192)  {ip_class = B;}
  else if (class_byte < 224)  {ip_class = C;}
  else if (class_byte < 240)  {ip_class = D;}
  else if (class_byte < 255)  {ip_class = E;}

  input_ip->ip_class = ip_class;
  return 0;
}

// Returns whether ip1 and ip2 can connect with each other
bool are_connectable(IP *ip1, IP *ip2) {
  switch (ip1->ip_class) {
  case A:   // A class
    return ip1->addr[0] == ip2->addr[0];
  case B:   // B class
    return ip1->addr[0] == ip2->addr[0] && ip1->addr[1] == ip2->addr[1];
  case C:   // C class
    return ip1->addr[0] == ip2->addr[0] && ip1->addr[1] == ip2->addr[1] && ip1->addr[2] == ip2->addr[2];
  case D:   // Multicast
    return true;
  case E:   // Reserved
  case LOOPBACK:  // Loopback
    return false;
  }
  return true;  // Compiler complains otherwise
}

// Formats given IP for printing
char* format_ip(IP *ip, char *st_buffer) {
  sprintf(st_buffer, "%d.%d.%d.%d", ip->addr[0], ip->addr[1], ip->addr[2], ip->addr[3]);
  return st_buffer;
}

int main() {
  IP ip1, ip2;
  char ip1_fmt[MAX_IP_LEN], ip2_fmt[MAX_IP_LEN]; // Formatted counterparts

  const int ret1 = read_ip(&ip1);
  const int ret2 = read_ip(&ip2);

  if (ret1 < 0 || ret2 < 0) {
    fprintf(stderr, "One of the IPs is invalid (127...)");
    return -1;
  }
  const bool can_connect = are_connectable(&ip1, &ip2); // Get connectability

  format_ip(&ip1, ip1_fmt); format_ip(&ip2, ip2_fmt); // Format them for printing
  
  printf("\n%s%s%s%s\n", ip1_fmt, (can_connect ? " is " : " is NOT "), "connectable with ", ip2_fmt);
  return 0;
}