### Question:
IPUtils { public static final boolean isValidIpv4Subnet(String in) { String ip; String subnet; Matcher ipMatcher; if (in == null) { return false; } ipMatcher = subnetPattern.matcher(in); int subnetint; if (ipMatcher.find()) { ip = ipMatcher.group(1); subnet = ipMatcher.group(2); try { subnetint = Integer.parseInt(subnet); if (subnetint < 0 || subnetint > 32 || !isValidIpv4String(ip)) { return false; } return true; } catch (NumberFormatException e) { return false; } } return false; }  static boolean isValidIpv6String(String in); static final boolean isValidIpv4Subnet(String in); static boolean isValidIpv6Subnet(String in); static boolean isValidIpv4String(String in); static byte[] invBytes(byte[] mask); static byte[] opBytes(byte[] a1, byte[] a2, ByteStreamOperation bop); static byte[] rollMask(int mask, int addrSize); static boolean bytesEqual(byte[] a1, byte[] a2);  }### Answer:
@Test public void testAcceptValidIPv4Subnet() { assertTrue(IPUtils.isValidIpv4Subnet("192.168.3.51/28")); }
@Test public void testRejectSubnetOutOfRangeIPv4() { assertFalse(IPUtils.isValidIpv4Subnet("192.168.3.51/33")); assertFalse(IPUtils.isValidIpv4Subnet("192.168.3.51/-1")); }
@Test public void testRejectNullSubnetIPv4() { assertFalse(IPUtils.isValidIpv4Subnet(null)); }
@Test public void testRejectSubnetwithoutnumberIPv4() { assertFalse(IPUtils.isValidIpv4Subnet("192.168.3.51/")); }