Access lists perform packet filtering to control which packets move through the network and where. Such control can help limit network traffic and restrict the access of users and devices to the network. Access lists have many uses, and therefore many commands accept a reference to an access list in their command syntax. Access lists can be used to do the following:

• Filter incoming packets on an interface.
• Filter outgoing packets on an interface.
• Restrict the contents of routing updates.
• Limit debug output based on an address or protocol.
• Control virtual terminal line access.
• Identify or classify traffic for advanced features, such as congestion avoidance, congestion management, and priority and custom queuing.
• Trigger dial-on-demand routing (DDR) calls.

An access list is a sequential list consisting of a permit statement and a deny statement that apply to IP addresses and possibly upper-layer IP protocols. The access list has a name by which it is referenced. Many software commands accept an access list as part of their syntax.

An access list can be configured and named, but it is not in effect until the access list is referenced by a command that accepts an access list. Multiple commands can reference the same access list. An access list can control traffic arriving at the router or leaving the router, but not traffic originating at the router.

• The software tests the source or destination address or the protocol of each packet being filtered against the conditions in the access list, one condition (permit or deny statement) at a time.
• If a packet does not match an access list statement, the packet is then tested against the next statement in the list.
• If a packet and an access list statement match, the rest of the statements in the list are skipped and the packet is permitted or denied as specified in the matched statement. The first entry that the packet matches determines whether the software permits or denies the packet. That is, after the first match, no subsequent entries are considered.
• If the access list denies the address or protocol, the software discards the packet and returns an ICMP Host Unreachable message.
• If no conditions match, the packet is dropped. This is because each access list ends with an unwritten or implicit deny statement. That is, if the packet has not been permitted by the time it was tested against each statement, it is denied.
• Because the software stops testing conditions after the first match, the order of the conditions is critical. The same permit or deny statements specified in a different order could result in a packet being passed under one circumstance and denied in another circumstance.
• If an access list is referenced by name in a command, but the access list does not exist, all packets pass.
• Only one access list per interface, per protocol, per direction is allowed.
• Inbound access lists process packets arriving at the router. Incoming packets are processed before being routed to an outbound interface. An inbound access list is efficient because it saves the overhead of routing lookups if the packet is to be discarded because it is denied by the filtering tests. If the packet is permitted by the tests, it is then processed for routing. For inbound lists, permit means continue to process the packet after receiving it on an inbound interface; deny means discard the packet.
• Outbound access lists process packets before they leave the router. Incoming packets are routed to the outbound interface and then processed through the outbound access list. For outbound lists, permit means send it to the output buffer; deny means discard the packet.

• Create the access list before applying it to an interface. An interface with an empty access list applied to it permits all traffic.
• Another reason to configure an access list before applying it is because if you applied a nonexistent access list to an interface and then proceed to configure the access list, the first statement is put into effect, and the implicit deny statement that follows could cause you immediate access problems.
• Organize your access list so that more specific references in a network or subnet appear before more general ones.
• In order to make the purpose of individual statements more easily understood at a glance, you can write a helpful remark before or after any statement.

Source address and destination addresses are two of the most typical fields in an IP packet on which to base an access list. Specify source addresses to control packets from certain networking devices or hosts. Specify destination addresses to control packets being sent to certain networking devices or hosts.

Address filtering uses wildcard masking to indicate to the software whether to check or ignore corresponding IP address bits when comparing the address bits in an access list entry to a packet being submitted to the access list. By carefully setting wildcard masks, an administrator can select single or several IP addresses for permit or deny tests.

Wildcard masking for IP address bits uses the number 1 and the number 0 to specify how the software treats the corresponding IP address bits. A wildcard mask is sometimes referred to as an inverted mask because a 1 and 0 mean the opposite of what they mean in a subnet (network) mask.

• A wildcard mask bit 0 means check the corresponding bit value.
• A wildcard mask bit 1 means ignore that corresponding bit value.

If you do not supply a wildcard mask with a source or destination address in an access list statement, the software assumes a default wildcard mask of 0.0.0.0.

Unlike subnet masks, which require contiguous bits indicating network and subnet to be ones, wildcard masks allow noncontiguous bits in the mask.

You can filter packets based on transport layer information, such as whether the packet is a TCP, UDP, ICMP or IGMP packet.

The ability to apply sequence numbers to IP access list entries simplifies access list changes. Prior to the IP Access List Entry Sequence Numbering feature, there was no way to specify the position of an entry within an access list. If a user wanted to insert an entry (statement) in the middle of an existing list, all of the entries after the desired position had to be removed, then the new entry was added, and then all the removed entries had to be reentered. This method was cumbersome and error prone.

This feature allows users to add sequence numbers to access list entries and resequence them. When a user adds a new entry, the user chooses the sequence number so that it is in a desired position in the access list. If necessary, entries currently in the access list can be resequenced to create room to insert the new entry.

• For backward compatibility with previous releases, if entries with no sequence numbers are applied, the first entry is assigned a sequence number of 10, and successive entries are incremented by 10. The maximum sequence number is 2147483647. If the generated sequence number exceeds this maximum number, the following message is displayed:

	Exceeded maximum sequence number.

• If the user enters an entry without a sequence number, it is assigned a sequence number that is 10 greater than the last sequence number in that access list and is placed at the end of the list.
• If the user enters an entry that matches an already existing entry (except for the sequence number), then no changes are made.
• If the user enters a sequence number that is already present, the following error message is generated:

	Duplicate sequence number.

• If a new access list is entered from global configuration mode, then sequence numbers for that access list are generated automatically.
• Distributed support is provided so that the sequence numbers of entries in the Route Processor (RP) and line card (LC) are in synchronization at all times.
• Sequence numbers are not nvgened. That is, the sequence numbers themselves are not saved. In the event that the system is reloaded, the configured sequence numbers revert to the default sequence starting number and increment. The function is provided for backward compatibility with software releases that do not support sequence numbering.
• This feature works with named standard and extended IP access lists. Because the name of an access list can be designated as a number, numbers are acceptable.

1.    enable

2.    configure terminal

3.    ip access-list resequence access-list-name starting-sequence-number increment

4.    ip access-list {standard| extended} access-list-name

5.   Do one of the following:

6.   Do one of the following:

7.    Repeat Step 5 and/or Step 6 as necessary, adding statements by sequence number where you planned. Use the no sequence-number command to delete an entry.

8.    end

9.    show ip access-lists access-list-name

Router# show ip access-lists kmd1
Standard IP access list kmd1
100 permit 10.4.4.0, wildcard bits 0.0.0.255
105 permit 10.5.5.0, wildcard bits 0.0.0.255
115 permit 10.0.0.0, wildcard bits 0.0.0.255
130 permit 10.5.5.0, wildcard bits 0.0.0.255
145 permit 10.0.0.0, wildcard bits 0.0.0.255