input: Write a python function to validate if a given string is a valid IP address.
  assert is_ip_valid("192.168.0.1") == True
  assert is_ip_valid("256.256.256.256") == False
  assert is_ip_valid("abc.def.ghi.jkl") == False
  assert is_ip_valid("123.45.67.89") == True
output: def is_ip_valid(ip):
      parts = ip.split('.')
      if len(parts) != 4:
          return False
      for part in parts:
          if not part.isdigit():
              return False
          num = int(part)
          if num < 0 or num > 255:
              return False
      return True