![]() A 0 indicates that the address bits must be considered (exact match) a 1 in the mask is a do not care. When the value of the mask is broken down into binary (0s and 1s), the results determine which address bits to consider when traffic is processed. This is sometimes called an inverse mask or a wildcard mask. ![]() Masks for IP ACLs are the reverse, for example, mask 0.0.0.255. Masks in order to configure IP addresses on interfaces start with 255 and have the large values on the left side, for example, IP address 10.165.202.129 with a 255.255.255.224 mask. Masks are used with IP addresses in IP ACLs to specify what must be permitted and denied. A discussion of these functions is outside the scope of this document. Note: ACLs can also be used to define traffic to Network Address Translate (NAT), encrypt or filter non-IP protocols such as AppleTalk or IPX. Note: Only registered Cisco users can access internal information. RFC 1918 contains address allocation for private Internets, IP addresses which must not normally be seen on the Internet. Note: RFC 1700 contains assigned numbers of well-known ports. It also contains brief descriptions of the IP ACL types, feature availability, and an example of use in a network. This document describes how IP access control lists (ACLs) can filter network traffic. Refer to Cisco Technical Tips Conventionsfor more information on document conventions. If your network is live, ensure that you understand the potential impact of any command. All of the devices used in this document started with a cleared (default) configuration. The information in this document was created from the devices in a specific lab environment. This is noted in the discussion of each type. Some of these are present since Cisco IOS Software Releases 8.3 and others were introduced in later software releases. ![]() This document discusses various types of ACLs. This is noted under each access list feature. The concepts discussed are present in Cisco IOS ®Software Releases 8.3 or later. There are no specific prerequisites for this document. This document describes various types of IP Access Control Lists (ACLs) and how they can filter network traffic. ![]()
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