IPv4 Access Control Lists (ACLs)

Planning an ACL Application

Explicitly Permitting Any IPv4 Traffic: Entering a permit any or a permit ip any any ACE in an ACL permits all IPv4 traffic not previously permitted or denied by that ACL. Any ACEs listed after that point do not have any effect.

Explicitly Denying Any IPv4 Traffic: Entering a deny any or a deny ip any any ACE in an ACL denies all IPv4 traffic not previously per­ mitted or denied by that ACL. Any ACEs after that point have no effect.

Replacing One ACL with Another Using the Same Application: For a specific interface, the most recent ACL assignment using a given application replaces any previous ACL assignment using the same application on the same interface.

Static Port ACLs: These are applied per-port, per port-list, or per static trunk. Adding a port to a trunk applies the trunk’s ACL config­ uration to the new member. If a port is configured with an ACL, the ACL must be removed before the port is added to the trunk. Also, removing a port from an ACL-configured trunk removes the ACL configuration from that port.

How an ACE Uses a Mask To Screen Packets forMatches

When the switch applies an ACL to IPv4 traffic, each ACE in the ACL uses an IPv4 address and ACL mask to enforce a selection policy on the packets being screened. That is, the mask determines the range of IPv4 addresses (SA only or SA/DA) that constitute a match between the policy and a packet being screened.

What Is the Difference Between Network (or Subnet)

Masks and the Masks Used with ACLs?

In common IPv4 addressing, a network (or subnet) mask defines which part of the address to use for the network number and which part to use for the hosts on the network. For example:

Address

Mask

Network Address

Host Address

10.38.252.195

255.255.255.0

first three octets

The fourth octet.

10.38.252.195

255.255.248.0

first two octets and the left-

The right most three bits of the

 

 

most five bits of the third octet

third octet and all bits in the

 

 

 

fourth octet.

 

 

 

 

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