diff --git a/lib/rex/socket.rb b/lib/rex/socket.rb
index 6a0dca6262..11a48ffc82 100644
--- a/lib/rex/socket.rb
+++ b/lib/rex/socket.rb
@@ -201,6 +201,18 @@ module Socket
 		addr_ntoi(resolv_nbo(host))
 	end
 
+	def self.resolv_to_cidr(mask)
+		mask_i = resolv_nbo_i(mask)
+		cidr = nil
+		0.upto(32) do |i|
+			if ((1 << i)-1) << (32-i) == mask_i
+				cidr = i
+				break
+			end
+		end
+		return cidr
+	end
+
 	#
 	# Resolves a host to a dotted address.
 	#
diff --git a/scripts/meterpreter/autoroute.rb b/scripts/meterpreter/autoroute.rb
index 248277f41f..b50eefcaf0 100644
--- a/scripts/meterpreter/autoroute.rb
+++ b/scripts/meterpreter/autoroute.rb
@@ -124,7 +124,13 @@ def validate_cmd(subnet=nil,netmask=nil)
 	end
 
 	unless(check_ip(subnet))
-		print_error "Subnet invalid"
+		print_error "Subnet invalid (must be IPv4)"
+		usage
+		return false
+	end
+
+	if(netmask and !(Rex::Socket.resolv_to_cidr(netmask)))
+		print_error "Netmask invalid (must define contiguous IP addressing)"
 		usage
 		return false
 	end