+# src_addr[:port][/src_masklen] [dst_addr[:port][/dst_masklen]]
+#
+# Where src_addr and dst_addr are IPv4 addresses in dotted
+# decimal notation or valid DNS names, port is a port number, and
+# src_masklen and dst_masklen are subnet masks in CIDR notation,
+# i.e. integer values from 2 to 30 representing the length
+# (in bits) of the network address. The masks and the whole
+# destination part are optional.