diff --git a/cps/cli.py b/cps/cli.py
index de12be5a..e76a12cc 100644
--- a/cps/cli.py
+++ b/cps/cli.py
@@ -1,7 +1,5 @@
-#!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
-
 #   This file is part of the Calibre-Web (https://github.com/janeczku/calibre-web)
 #     Copyright (C) 2018 OzzieIsaacs
 #
@@ -22,50 +20,17 @@ from __future__ import division, print_function, unicode_literals
 import sys
 import os
 import argparse
+import socket
 
 from .constants import CONFIG_DIR as _CONFIG_DIR
 from .constants import STABLE_VERSION as _STABLE_VERSION
 from .constants import NIGHTLY_VERSION as _NIGHTLY_VERSION
 
-VALID_CHARACTERS = 'ABCDEFabcdef:0123456789'
-
-ipv6 = False
-
 
 def version_info():
     if _NIGHTLY_VERSION[1].startswith('$Format'):
         return "Calibre-Web version: %s - unkown git-clone" % _STABLE_VERSION['version']
-    else:
-        return "Calibre-Web version: %s -%s" % (_STABLE_VERSION['version'],_NIGHTLY_VERSION[1])
-
-
-def validate_ip4(address):
-    address_list = address.split('.')
-    if len(address_list) != 4:
-        return False
-    for val in address_list:
-        if not val.isdigit():
-            return False
-        i = int(val)
-        if i < 0 or i > 255:
-            return False
-    return True
-
-
-def validate_ip6(address):
-    address_list = address.split(':')
-    return (
-        len(address_list) == 8
-        and all(len(current) <= 4 for current in address_list)
-        and all(current in VALID_CHARACTERS for current in address)
-    )
-
-
-def validate_ip(address):
-    if validate_ip4(address) or ipv6:
-        return address
-    print("IP address is invalid. Exiting")
-    sys.exit(1)
+    return "Calibre-Web version: %s -%s" % (_STABLE_VERSION['version'], _NIGHTLY_VERSION[1])
 
 
 parser = argparse.ArgumentParser(description='Calibre Web is a web app'
@@ -95,8 +60,8 @@ if sys.version_info < (3, 0):
         args.s = args.s.decode('utf-8')
 
 
-settingspath    = args.p or os.path.join(_CONFIG_DIR, "app.db")
-gdpath          = args.g or os.path.join(_CONFIG_DIR, "gdrive.db")
+settingspath = args.p or os.path.join(_CONFIG_DIR, "app.db")
+gdpath       = args.g or os.path.join(_CONFIG_DIR, "gdrive.db")
 
 # handle and check parameter for ssl encryption
 certfilepath = None
@@ -108,7 +73,7 @@ if args.c:
         print("Certfilepath is invalid. Exiting...")
         sys.exit(1)
 
-if args.c is "":
+if args.c == "":
     certfilepath = ""
 
 if args.k:
@@ -122,15 +87,26 @@ if (args.k and not args.c) or (not args.k and args.c):
     print("Certfile and Keyfile have to be used together. Exiting...")
     sys.exit(1)
 
-if args.k is "":
+if args.k == "":
     keyfilepath = ""
 
 # handle and check ipadress argument
-if args.i:
-    ipv6 = validate_ip6(args.i)
-    ipadress = validate_ip(args.i)
-else:
-    ipadress = None
+ipadress = args.i or None
+if ipadress:
+    try:
+        # try to parse the given ip address with socket
+        if hasattr(socket, 'inet_pton'):
+            if ':' in ipadress:
+                socket.inet_pton(socket.AF_INET6, ipadress)
+            else:
+                socket.inet_pton(socket.AF_INET, ipadress)
+        else:
+            # on windows python < 3.4, inet_pton is not available
+            # inet_atom only handles IPv4 addresses
+            socket.inet_aton(ipadress)
+    except socket.error as err:
+        print(ipadress, ':', err)
+        sys.exit(1)
 
 # handle and check user password argument
-user_password   = args.s or None
+user_password = args.s or None
diff --git a/cps/config_sql.py b/cps/config_sql.py
index b99351cd..0089ad0f 100644
--- a/cps/config_sql.py
+++ b/cps/config_sql.py
@@ -136,9 +136,6 @@ class _ConfigSQL(object):
     def get_config_ipaddress(self):
         return cli.ipadress or ""
 
-    def get_ipaddress_type(self):
-        return cli.ipv6
-
     def _has_role(self, role_flag):
         return constants.has_flag(self.config_default_role, role_flag)
 
diff --git a/cps/constants.py b/cps/constants.py
index 8d0002f1..2bedb282 100644
--- a/cps/constants.py
+++ b/cps/constants.py
@@ -128,4 +128,3 @@ NIGHTLY_VERSION[1] = '$Format:%cI$'
 
 # clean-up the module namespace
 del sys, os, namedtuple
-
diff --git a/cps/server.py b/cps/server.py
index fbb14b84..e5fe78e4 100644
--- a/cps/server.py
+++ b/cps/server.py
@@ -43,7 +43,14 @@ from . import logger
 log = logger.create()
 
 
-class WebServer:
+
+def _readable_listen_address(address, port):
+    if ':' in address:
+        address = "[" + address + "]"
+    return '%s:%s' % (address, port)
+
+
+class WebServer(object):
 
     def __init__(self):
         signal.signal(signal.SIGINT, self._killServer)
@@ -55,14 +62,12 @@ class WebServer:
         self.app = None
         self.listen_address = None
         self.listen_port = None
-        self.IPV6 = False
         self.unix_socket_file = None
         self.ssl_args = None
 
     def init_app(self, application, config):
         self.app = application
         self.listen_address = config.get_config_ipaddress()
-        self.IPV6 = config.get_ipaddress_type()
         self.listen_port = config.config_port
 
         if config.config_access_log:
@@ -77,8 +82,7 @@ class WebServer:
         keyfile_path = config.get_config_keyfile()
         if certfile_path and keyfile_path:
             if os.path.isfile(certfile_path) and os.path.isfile(keyfile_path):
-                self.ssl_args = {"certfile": certfile_path,
-                                  "keyfile": keyfile_path}
+                self.ssl_args = dict(certfile=certfile_path, keyfile=keyfile_path)
             else:
                 log.warning('The specified paths for the ssl certificate file and/or key file seem to be broken. Ignoring ssl.')
                 log.warning('Cert path: %s', certfile_path)
@@ -106,32 +110,33 @@ class WebServer:
         if os.name != 'nt':
             unix_socket_file = os.environ.get("CALIBRE_UNIX_SOCKET")
             if unix_socket_file:
-                output = "socket:" + unix_socket_file + ":" + str(self.listen_port)
-                return self._make_gevent_unix_socket(unix_socket_file), output
+                return self._make_gevent_unix_socket(unix_socket_file), "unix:" + unix_socket_file
 
         if self.listen_address:
-            return (self.listen_address, self.listen_port), self._get_readable_listen_address()
+            return (self.listen_address, self.listen_port), None
 
         if os.name == 'nt':
             self.listen_address = '0.0.0.0'
-            return (self.listen_address, self.listen_port), self._get_readable_listen_address()
+            return (self.listen_address, self.listen_port), None
 
-        address = ('', self.listen_port)
         try:
+            address = ('::', self.listen_port)
             sock = WSGIServer.get_listener(address, family=socket.AF_INET6)
-            output = self._get_readable_listen_address(True)
         except socket.error as ex:
             log.error('%s', ex)
             log.warning('Unable to listen on "", trying on IPv4 only...')
-            output = self._get_readable_listen_address(False)
+            address = ('', self.listen_port)
             sock = WSGIServer.get_listener(address, family=socket.AF_INET)
-        return sock, output
+
+        return sock, _readable_listen_address(*address)
 
     def _start_gevent(self):
         ssl_args = self.ssl_args or {}
 
         try:
             sock, output = self._make_gevent_socket()
+            if output is None:
+                output = _readable_listen_address(self.listen_address, self.listen_port)
             log.info('Starting Gevent server on %s', output)
             self.wsgiserver = WSGIServer(sock, self.app, log=self.access_logger, spawn=Pool(), **ssl_args)
             self.wsgiserver.serve_forever()
@@ -141,30 +146,18 @@ class WebServer:
                 self.unix_socket_file = None
 
     def _start_tornado(self):
-        log.info('Starting Tornado server on %s', self._get_readable_listen_address())
+        log.info('Starting Tornado server on %s', _readable_listen_address(self.listen_address, self.listen_port))
 
         # Max Buffersize set to 200MB            )
         http_server = HTTPServer(WSGIContainer(self.app),
-                    max_buffer_size = 209700000,
-                    ssl_options=self.ssl_args)
+                                 max_buffer_size=209700000,
+                                 ssl_options=self.ssl_args)
         http_server.listen(self.listen_port, self.listen_address)
-        self.wsgiserver=IOLoop.instance()
+        self.wsgiserver = IOLoop.instance()
         self.wsgiserver.start()
         # wait for stop signal
         self.wsgiserver.close(True)
 
-    def _get_readable_listen_address(self, ipV6=False):
-        if self.listen_address == "":
-            listen_string = '""'
-        else:
-            ipV6 = self.IPV6
-            listen_string = self.listen_address
-        if ipV6:
-            adress = "[" + listen_string + "]"
-        else:
-            adress = listen_string
-        return adress + ":" + str(self.listen_port)
-
     def start(self):
         try:
             if _GEVENT:
@@ -191,7 +184,7 @@ class WebServer:
         os.execv(sys.executable, arguments)
         return True
 
-    def _killServer(self, signum, frame):
+    def _killServer(self, ignored_signum, ignored_frame):
         self.stop()
 
     def stop(self, restart=False):
diff --git a/cps/services/simpleldap.py b/cps/services/simpleldap.py
index f9d0dfff..7feec202 100644
--- a/cps/services/simpleldap.py
+++ b/cps/services/simpleldap.py
@@ -29,10 +29,7 @@ _ldap = LDAP()
 
 
 def init_app(app, config):
-    global _ldap
-
     if config.config_login_type != constants.LOGIN_LDAP:
-        _ldap = None
         return
 
     app.config['LDAP_HOST'] = config.config_ldap_provider_url