diff --git a/src/isinetaddr.c b/src/isinetaddr.c
index 4de8483..87146af 100644
--- a/src/isinetaddr.c
+++ b/src/isinetaddr.c
@@ -2,24 +2,34 @@
 #include <string.h>
 #include <stdlib.h>
 #include <isinetaddr.h>
+#include <errno.h>
+
+static
+int inrange(char buf[4])
+{
+  char *err;
+  long r;
+  errno = 0;
+  r = strtol(buf, &err, 10);
+  return *err == '\0' && errno == 0 && (r >= 0 && r <= 255);
+}
 
 int
 isinetaddr(const char *str)
 {
   char buf[4];
-  const char *err;
   int i = 0, j = 0, k = 0;
+  size_t len = strnlen(str, 16);
 
-  for(int l = 0; l < (int)strnlen(str, 16); l++) {
+  for(size_t l = 0; l < len; l++) {
     if (str[l] == '.') {
-      strtonum(buf, 0, 255, &err);
-      if (err != NULL) {
-        return 0;
-      } else {
+      if (inrange(buf)) {
         k = 0;
         j++;
         bzero(buf, 3);
         buf[3] = '\0';
+      } else {
+        return 0;
       }
     } else if (isdigit(str[l])) {
       buf[k++] = str[l];
@@ -31,3 +41,4 @@ isinetaddr(const char *str)
   }
   return j == 3 && i <= 12 ? 1 : 0;
 }
+