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Manage IP traffic as network access growsManage IP traffic as network access grows Filter packets as they pass through the routerFilter packets as they pass through the router

Why Use Access Lists?Why Use Access Lists?


What are ACLs?What are ACLs?

ACLs are lists of conditions that are applied to traffic traveling across a router's interface.

These lists tell the router what types of packets to accept or deny.

Acceptance and denial can be based on specified conditions.

ACLs can be configured at the router to control access to a network or subnet.

Some ACL decision points are source and destination addresses, protocols, and upper-layer port numbers.


Reasons to Create ACLsReasons to Create ACLs

The following are some of the primary reasons to create ACLs:

Limit network traffic and increase network performance. Provide traffic flow control. Provide a basic level of security for network access. Decide which types of traffic are forwarded or blocked at the router interfacesFor example: Permit e-mail traffic to be routed, but block all telnet traffic. If ACLs are not configured on the router, all packets passing through the router will be allowed onto all parts of the network.


ACL’sACL’s

Different access list for TelnetDifferent access list for Telnet Implicit deny at bottomImplicit deny at bottom All restricted statements should be on firstAll restricted statements should be on first There are two typesThere are two types

StandardStandard ExtendedExtended


IP PacketIP Packet

SRC IP AddressDEST IP AddressProtocol typeSRC PortDEST Port


StandardChecks source addressPermits or denies entire protocol suite

ExtendedChecks source and destination addressGenerally permits or denies specific protocols

Types of Access Lists

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How to Identify Access ListsHow to Identify Access Lists

Standard IP lists (1-99) test conditions of all IP packets from Standard IP lists (1-99) test conditions of all IP packets from source addresses.source addresses.

Extended IP lists (100-199) test conditions of source and Extended IP lists (100-199) test conditions of source and destination addresses, specific TCP/IP protocols, and destination addresses, specific TCP/IP protocols, and destination ports.destination ports.

Standard IP lists (1300-1999) (expanded range).Standard IP lists (1300-1999) (expanded range). Extended IP lists (2000-2699) (expanded range).Extended IP lists (2000-2699) (expanded range).


Standard ACLsStandard ACLs

The full syntax of the standard ACL command is:

Router(config)#access-list access-list-number {deny | permit} source [source-wildcard ]

The no form of this command is used to remove a standard ACL. This is the syntax:Router(config)#no access-list access-list-number

Config# Access-list 1 deny 192.168.1.0 0.0.0.255Config# access-list 1 permit any


The ANY and HOST keywordThe ANY and HOST keyword

Access-list 1 permit 200.0.0.9 0.0.0.0Access-list 1 permit 200.0.0.9 0.0.0.0

OrOr

permit host 200.0.0.9permit host 200.0.0.9

Access-list 1 permit 0.0.0.0 255.255.255.255

Orpermit any


The ip access-group commandThe ip access-group command

{ in | out }


Extended ACLsExtended ACLs

Extended ACLs are used more often than standard ACLs because they provide a greater range of control.

Extended ACLs check the source and destination packet addresses as well as being able to check for protocols and port numbers.

Logical operations may be specified such as, equal (eq), not equal (neq), greater than (gt), and less than (lt), that the extended ACL will perform on specific protocols.

Extended ACLs use an access-list-number in the range 100 to 199 (also from 2000 to 2699 in recent IOS).


Extended ACL SyntaxExtended ACL Syntax


Deny FTPDeny FTP

access-list 101 deny tcp any any eq 21

access-list 101 permit ip any any

or

access-list 101 deny tcp any any eq ftp

access-list 101 permit ip any any


RulesRulesFor extended access list apply near For extended access list apply near

to the sourceto the sourceFor standard access list apply near to For standard access list apply near to

the destinationthe destination


Verify Access ListVerify Access List


Basic Rules for ACLsBasic Rules for ACLs

Standard IP access lists should be applied closest to the destination. Extended IP access lists should be applied closest to the source. Use the inbound or outbound interface reference as if looking at the port from inside the router. Statements are processed sequentially from the top of list to the bottom until a match is found, if no match is found then the packet is denied. There is an implicit deny at the end of all access lists. This will not appear in the configuration listing. Access list entries should filter in the order from specific to general. Specific hosts should be denied first, and groups or general filters should come last. Never work with an access list that is actively applied. New lines are always added to the end of the access list. A no access-list x command will remove the whole list. It is not possible to selectively add and remove lines with numbered ACLs. Outbound filters do not affect traffic originating from the local router.

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