CCNA Access Lists

CCNA Guide to Cisco Networking Fundamentals

Chapter 10Access Lists

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Objectives

• Describe the usage and rules of access lists

• Establish standard IP access lists

• Produce extended IP access lists

• Apply access lists to interfaces

• Monitor and verify access lists

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Objectives (continued)

• Create named access lists

• Use Security Device Manager to create standard and extended IP access lists

• Use Security Device Manager to create a router firewall

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Access Lists: Usage and Rules

• Access lists– Permit or deny statements that filter traffic based on

the source address, destination address, protocol type, and port number of a packet

– Available for IP, IPX, AppleTalk, and many other protocols

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Access List Usage

• You can create a standard access list that examines a packet for the packet’s source header information

• deny any statement– Implicitly blocks all packets that do not meet the

requirements of the access list– Exists even though it is not shown as part of the

access list

• With careful planning, you can create access lists that control which traffic crosses particular links– And which segments of your network will have access

to others

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Access List Usage (continued)

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Problems with Access Lists

• Lack of planning is one of the most common problems associated with access lists

• The need to enter the list sequentially into the router also presents problems– You cannot move individual statements once they are

entered– When making changes, you must remove the list,

using the no access-list [list number] command, and then retype the commands

• Access lists begin working the second they are applied to an interface

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Access List Rules

• Example of the structure of a standard IP access list:RouterA(config)#access-list 1 deny 172.22.5.2 0.0.0.0

RouterA(config)#access-list 1 deny 172.22.5.3 0.0.0.0

RouterA(config)# access-list 1 permit any

• Router applies each line in the order in which you type it into the access list

• The no access-list [list #] command is used to remove an access list

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Access List Rules (continued)

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Access List Rules (continued)

• As a general rule, the lines with the most potential matches should be first in the list– So that packets will not undergo unnecessary

processing

• You should avoid unnecessarily long access lists

• After you create access lists, you must apply them to interfaces so they can begin filtering traffic– You apply a list as either an outgoing or an incoming

filter

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Access List Rules (continued)• In summary, all access lists follow these rules:

– Routers apply lists sequentially in the order in which you type them into the router

– Routers apply lists to packets sequentially, from the top down, one line at a time

– Packets are processed only until a match is made– Lists always end with an implicit deny– Access lists must be applied to an interface as either inbound

or outbound traffic filters– Only one list, per protocol, per direction can be applied to an

interface– Access lists are effective as soon as they are applied

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Standard IP Access Lists

• Standard IP access lists– Filter network traffic based on the source IP address

only– Using a standard IP access list, you can filter traffic

by a host IP, subnet, or a network address

• Configure standard IP access lists:– access-list [list #] [permit|deny] [source address] [source wildcard mask]

• Routers use wildcards to determine which bits in an address will be significant

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Standard IP Access Lists (continued)

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Standard IP Access Lists (continued)

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Standard IP Access Lists (continued)

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Standard IP Access Lists (continued)

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Standard IP Access Lists (continued)

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Standard IP Access List Examples

• Standard IP access lists permit or deny packets based only on the source address– Addresses can be a single host address, a subnet

address, or a full network address

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Standard IP Access List Examples (continued)

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Standard IP Access List Examples (continued)

• Correct placement of a list is imperative

• To view the access lists defined on your router, use the show access-lists command– For IP access lists you could also use the show ip access-lists command

• If you decide that an access list needs to be removed from an interface– You can remove it with the no ip access-group [list #] command

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Standard IP Access List Examples (continued)

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Standard IP Access List Examples (continued)

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Standard IP Access List Examples (continued)

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Standard IP Access List Examples (continued)

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Standard IP Access List Examples (continued)

• Application of the list as an outbound filter on FastEthernet0/0– See Figure 10-15

• Use the show access-lists or show ip access-lists command followed by the show ip interface command– To verify that the list has been entered and applied

correctly

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Standard IP Access List Examples (continued)

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Standard IP Access List Examples (continued)

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Monitoring Standard IP Access Lists

• Three main commands are available for monitoring access lists on your router– show access-lists– show ip access-lists– show interfaces or show ip interface

• Use the no access-list [list #] command to remove the list

• Use the no ip accessgroup [list #][direction] command to remove the application of the list

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Extended IP Access Lists

• Extended IP access lists– Can filter by source IP address, destination IP

address, protocol type, and application port number– This granularity allows you to design extended IP

access lists that:• Permit or deny a single type of IP protocol

• Filter by a particular port of a particular protocol

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Extended IP Access Lists (continued)

• To configure extended IP access lists, you must create the list and then apply it to an interface using the following syntax– access-list [list #] [permit|deny] [protocol] [source IP address] [source wildcard mask] [operator] [port] [destination IP address] [destination wildcard mask] [operator] [port] [log]

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Extended IP Access List Examples

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Extended IP Access List Examples (continued)

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The “Established” Parameter

• Established parameter– Permits traffic from any host on any network to any

destination, as long as the traffic was in response to a request initiated inside the network

• Example:access-list 100 permit tcp any 15.0.0.0 0.255.255.255 established

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Monitoring Extended IP Access Lists

• The same commands used to monitor standard IP access lists are used to monitor extended IP access lists

• Extended IP lists keep track of the number of packets that pass each line of an access list– The clear access-list counters [list #]

command clears the counters– The no access-list [list#] command removes

the list – The no ip access-group [list#] [direction]

command removes the application of the list

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Monitoring Extended IP Access Lists

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Monitoring Extended IP Access Lists

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Using Named Lists

• Named access lists– In Cisco IOS versions 11.2 and above, names instead

of numbers can be used to identify lists

• To name a standard IP access list, use the following syntax:

RouterC(config)#ip access-list standard [name]

• To name an extended IP access list, use the following syntax:

RouterC(config)#ip access-list extended [name]

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Using Named Lists (continued)

• Once the list is named, the permit or deny statement is entered

• The commands follow the same syntax as unnamed lists– The beginning part of the command is not included

• To apply a standard IP named list to an interface, the syntax is:

RouterC(config-if)#ip access-group [name] [in | out]

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Using Named Lists (continued)

• Advantages:– Allows you to maintain security by using an easily

identifiable access list– Removes the limit of 100 lists per filter type– With named access lists lines can be selectively

deleted in the ACL– Named ACLs provide greater flexibility to network

administrators who work in environments where large numbers of ACLs are needed

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Controlling VTY Line Access

• Access lists are used for both traffic flow and security

• One useful security feature of access lists is restricting access to telnet on your router– By controlling VTY line access

• You must first create a standard IP access list that permits the management workstation

RouterA(config)#access-list 12 permit 192.168.12.12 0.0.0.0

• Then, it must be applied to the VTY linesaccess-class [acl #] in | out

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Controlling VTY Line Access (continued)

• To apply access list 12 to the VTY lines, use the following command:RouterA(config)#line vty 0 4

RouterA(config-line)#access-class 12 in

• The commands to restrict access to the VTY lines to network 192.168.12.0/24 only are:RouterA(config)#access-list 13 permit 192.168.12.0 0.0.0.255

RouterA(config)#line vty 0 4

RouterA(config-line)#access-class 13 in

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Using Security Device Manager to Create Access Control Lists

• Using the SDM, an administrator can accomplish all the tasks that formerly required use of the CLI interface

• SDM allows you to easily create a standard or an extended access list or, as it is known in the SDM, an Access Control List (ACL)

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Using Security Device Manager to Create a Router Firewall

• Unlike the CLI, the SDM allows a router to be configured as a firewall

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Using Security Device Manager to Create a Router Firewall (continued)

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Using Security Device Manager to Create a Router Firewall (continued)

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Summary

• Access lists are one of the most important IOS tools for controlling network traffic and security

• Access lists are created in a two-step process

• All access lists are created sequentially and applied sequentially to all packets that enter an interface where the list is applied

• By default, access lists always end in an implicit deny any statement

• Only one access list per direction (inbound or outbound) per protocol can be applied to an interface

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Summary (continued)

• Standard IP access lists allow you to filter traffic based on the source IP address of a packet

• Extended IP access lists filter traffic based on source, destination, protocol type, and application type

• Access lists can be used to restrict telnet by controlling VTY line access

• Ranges of numbers represent all access lists

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Summary (continued)

• The SDM can be used to configure both standard and extended ACLs via the Additional Tasks configuration tab

• The SDM can be used to configure a router as either a Basic or Advanced firewall

• The main difference between a Basic and Advanced firewall is the ability to configure DMZ interfaces in the Advanced firewall setup wizard