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Chapter 12
Using Access List
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Chapter Objectives
At the end of this Chapter you will be able to:

Introduce ACL concepts

Explain the types of ACL

Configure ACL

Introduce wildcard masks

Verify ACL
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Access Control List - I

Access Control List (ACL) is a set of commands, which are grouped
together to filter the traffic that enters or leaves an interface

The functions of ACLs include the following:

Reduce the network traffic and increase the network
performance

Control the flow of traffic in the network

Provide security for network access

Make decision about the type of network to be forwarded or
restricted on an interface
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Access Control List - II



You must also specify one of the following directions in which the
traffic should be filtered:

Inbound

Outbound
The Access lists are available in the following two types:

Numbered ACL

Named ACL
Each of the below ACLs supports two types:

Standard ACL

Extended ACL
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ACL Processing

ACL are statements, which are grouped together by using a name or
number

When ACL process a packet on the router from the group of
statements, the router performs a number of steps to find a match for
the ACL statements

The router processes each ACL in the top-down approach

If the router is not able to find a match of packet to the first ACL
statement then the router continues with the next statement,
following the same process
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Configuring ACL

You must know the guidelines to configure the ACL

You can use the access-list command to create an ACL

The syntax to create an ACL is:
access-list ACL_# permit|deny conditions

To activate ACL on the interface, enter the following at the command
prompt:
interface type slot_#|port_#
ip access-group ACL_# in|out
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ACL Numbers and Protocol Types - I
Type
Numbers
IP Standard
1-99, 1300-1999
Standard Vines
1-99
IP Extended
100-199, 2000-2699
Extended Vines
100-199
Bridging type code (layer-2)
200-299
DECnet
300-399
Standard XNS
400-499
Extended XNS
500-599
Apple Talk
600-699
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ACL Numbers and Protocol Types - II
Bridging MAC address and vendor code
700-799
IPX Standard
800-899
IPX Extended
900-999
IPX SAP filters
1000-1099
Extended transparent bridging
1100-1199
IPX NLSP
1200-1299
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Wildcard Masking

Wildcard masks can be used for matching a range of IP addresses in
ACL, instead of manually entering it

A wildcard mask of 0 or 1 in the bit position implies the following:

0 - Specifies that the bit position of the ACL statement
address must match the bit position in the IP address of the
packet that is to be examined

1 - Specifies that the bit position of the ACL statement
address does not have to match the bit position in the IP
address of the packet that is to be examined
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Subnet Masks and Wildcard Masks Values
Bit Value
Subnet Mask
Wildcard Mask
0
Host component
Must match
1
Network component
Ignore
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Standard Numbered ACL
• Standard IP access lists filter network traffic by examining the
source IP address in a packet.
• It always applies near to Destination.
• Range of standard IP access list is 1–99.
• After choose the access-list number, you need to decide
whether you’re creating a Permit or deny statement.
You can use any parameter to permit or deny any host or
network you can use an IP address to specify either a single
host or a range of them, or you can use the host command to
specify a specific host only.
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Standard Numbered ACL

You can create an entry in a standard numbered IP ACL using the
access-list command.The syntax of this command is:
access-list 1-99|1300-1999 permit|deny source_address
[wildcard_mask] [log]

After creating the standard ACL, you must activate it on the router's
interface

The ip access-group command below enables you to activate the ACL
on the interface:
ip access-group ACL_number in|out
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Extended Numbered ACLs
•Extended access lists allow you to specify source and destination
address as well as the protocol and port number.
•It always applies near to Source.
•Range of Extended IP access list is 100–199.
•After choose the access-list number, you need to decide whether
you’re creating a Permit or deny statement.
•You can effectively allow users access to a physical LAN and stop
them from accessing specific hosts—or even specific services on
those hosts.
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Extended Numbered ACLs

The extended numbered ACLs are more flexible as compared to the
standard ACLs

The syntax of the command to configure an extended numbered ACL is:

access-list 100-199|2000-2699 permit|deny IP_protocol
source_address source_wildcard mask [protocol_information]
destination_address destination_wildcard_mask [protocol_information]
[log]
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TCP and UDP

The extended ACL can filter the traffic on IP protocols such as TCP and
UDP

The syntax of the command that allows you to configure an extended
ACL for TCP or UDP is:
access-list 100-199|2000-2699 permit|deny tcp|udp source_address
source_wildcard_mask [operator source_port_#] destination address
destination_wildcard_mask [operator
destination_port_#][established][log]
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Operators for TCP and UDP Connection
Operator
Description
lt
Less than
gt
Greater than
neq
Not equal to
eq
Equal to
range
Range of port numbers
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TCP Port Names and Numbers
Name
Command Parameter
Number
FTP Data
ftp-data
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FTP Control
ftp
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Telnet
telnet
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SMTP
smtp
25
WWW
www
80
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UDP Port Names and Numbers
Name
Command Parameter
Number
DNS Query
dns
53
TFTP
tftp
69
SNMP
snmp
161
IP RIP
Rip
520
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Controlling Telnet Access to a Router


The standard IP ACLs controls the telnet access to the router. The
access-class command enables you to accomplish this. The syntax of
this command is:
access-class standard_ACL_# in|out

Cisco routers enable you to restrict telnet access for the normal users

You can configure your router to allow telnet access only for the
networks administrators
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Named ACL’s

Named ACLs support both IP and IPX protocols

The ip access-list allows you to create a named ACL

The syntax for this command is:
ip access-list standard/extended ACL_name

After creating a named ACL, you have to activate it on the router
interface

The ip access-group command given below enables you to activate the
named ACL:
ip access-group ACL_name in|out
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Verifying and Monitoring Access Control List

To view the ACLs that are activated on the router’s interfaces, use the
following command:
show ip interfaces

Cisco allows you to monitor each statement in the ACL. To view the
statements in the ACL, use the following commands:
show access-lists [ACL_#_or_name]
show ip access-list [ACL_#_or_name]
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Editing Entries

You may need to add, delete or modify an entry in the ACL

In a numbered ACL, you cannot delete a specific entry

The no access-list command allows you to delete an ACL, The syntax of
the command is:
no access-list number

The no access-list ACL_# command deletes the entire list
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Summary - I

The access control list is a set of commands, which are grouped together,
to filter the traffic that enters or leaves an interface

ACL are the statements, which are grouped together by using a name or
number. When ACL process a packet on the router from the group of
statements, the router performs a number of steps to find a match for
the ACL statements

After building IP ACL, you have to apply it to a process in the IOS

The extended numbered ACL’s are more flexible as compared to the
standard ACLs
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Summary - II

If the router locates a match between the packet and statement then
the router executes one of the two actions that are included with the
statement. The actions are:

Permit

Deny

A need may arise to add, delete or modify an entry in the ACL. You
cannot delete a specific entry in an ACL

To handle the IP addresses in ACL statements, wildcard masks can be
used for matching a range of address instead of manually entering it
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Summary - III

A wildcard is similar to an inverted subnet mask. If you want to match
on any address in a subnet or network then you have to take the
subnet mask and invert the bit values

You must consider the following while placing the ACLs:

Standard ACL - Needs to be placed close to the destination
device that you want to prevent the source from reaching.

Extended ACL - Needs to be placed close to the source
device. The extended ACL can filter on both the source as
well as destination address
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THANK YOU
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