IPValidator.java

package IPValidator;

public class IPValidator {

	private static int index;
	private static int dotIndex;
	private static int intBufferValue;
	private static int[] intBuffer;

	public static boolean isIPValid(String IP) {

		index = 0;
		dotIndex = 0;
		intBuffer = new int[3];

		for (char currentChar : IP.toCharArray()) {
			index += 1;
			if (currentChar == '.') {
				if (index == 4) {
					// reached eos so checking if segment is valid
					intBufferValue = intBuffer[0]*100+intBuffer[1]*10+intBuffer[2];
					if (intBufferValue > 255 || intBufferValue < 0) {
					// segment must be between 0 and 255
						return false;
					}
				}
				
				index = 0;
				dotIndex += 1;
				intBuffer = new int[3];
				
			} else if (Character.isDigit(currentChar)) {
				if (index == 4) {
				// can't have more than 4 digits in a segment
					return false;
				} else {
				// it is valid, add it to the buffer
					intBuffer[index-1] = Character.getNumericValue(currentChar);
				}
			} else {
			// letters can't be in IP's
				return false;
			}
		}
		if (dotIndex != 3) {
		// IP must have 3 dots, no more or less
			return false;
		}
		return true;
	}
}