diff --git a/server/filter.py b/server/filter.py
index 840c5316..0e5f9ecb 100644
--- a/server/filter.py
+++ b/server/filter.py
@@ -288,6 +288,9 @@ class Filter(JailThread):
 			# IP address without CIDR mask
 			if len(s) == 1:
 				s.insert(1, '32')
+			elif "." in s[1]: # 255.255.255.0 style mask
+				s[1] = len(re.search(
+					"(?<=b)1+", bin(DNSUtils.addr2bin(s[1]))).group())
 			s[1] = long(s[1])
 			try:
 				a = DNSUtils.cidr(s[0], s[1])
diff --git a/testcases/filtertestcase.py b/testcases/filtertestcase.py
index 30397d67..f1a9bcd2 100644
--- a/testcases/filtertestcase.py
+++ b/testcases/filtertestcase.py
@@ -188,6 +188,15 @@ class IgnoreIP(LogCaptureTestCase):
 		self.assertFalse(self.filter.inIgnoreIPList('192.168.1.255'))
 		self.assertFalse(self.filter.inIgnoreIPList('192.168.0.255'))
 
+	def testIgnoreIPMask(self):
+		self.filter.addIgnoreIP('192.168.1.0/255.255.255.128')
+		self.assertTrue(self.filter.inIgnoreIPList('192.168.1.0'))
+		self.assertTrue(self.filter.inIgnoreIPList('192.168.1.1'))
+		self.assertTrue(self.filter.inIgnoreIPList('192.168.1.127'))
+		self.assertFalse(self.filter.inIgnoreIPList('192.168.1.128'))
+		self.assertFalse(self.filter.inIgnoreIPList('192.168.1.255'))
+		self.assertFalse(self.filter.inIgnoreIPList('192.168.0.255'))
+
 	def testIgnoreInProcessLine(self):
 		self.filter.addIgnoreIP('192.168.1.0/25')
 		self.filter.addFailRegex('<HOST>')