Creating an IP Access List and Applying It to an Interface

IP access lists provide many benefits for securing a network and achieving nonsecurity goals, such as determining quality of service (QoS) factors or limiting debug command output. This module describes how to create standard, extended, named, and numbered IP access lists. An access list can be referenced by a name or a number. Standard access lists filter on only the source address in IP packets. Extended access lists can filter on source address, destination address, and other fields in an IP packet.

After you create an access list, you must apply it to something in order for it to have any effect. This module describes how to apply an access list to an interface. However, there are many other uses for access lists, which are mentioned in this module and described in other modules and in other configuration guides for various technologies.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Restrictions for Creating an IP Access List and Applying It to an Interface

The following restrictions apply when configuring IPv4 and IPv6 access control lists (ACLs)

  • Application control engine (ACE)-specific counters are not supported.

  • Layer 3 IPv4 and IPv6 ACLs are not supported on the same interface.

  • MAC ACLs are not supported on Ethernet flow points (EFPs) or trunk EFP interfaces to which Layer 3 IPv4 or IPv6 ACLs are applied.

  • A maximum of 500 ACEs per ACL are supported.

  • IPv4 and IPv6 ACLs are not currently supported on EFP interfaces. IPv4 and IPv6 ACLs are supported on physical interfaces, bridge-domain interfaces, and port-channel interfaces.

  • Layer 4 port-range functionality expands into Ternary Content-Addressable Memory (TCAM). IPv4 ACL scale is limited to 1K TCAM, Layer 2 ACL scale is limited to 1K TCAM entries.

  • ACL counters or statistics are not supported in Cisco ASR 900 RSP3 Module.

  • Object-groups are not supported with IP ACLs.

  • IPv6 ACL is not supported in Cisco ASR 900 RSP3 Module.

  • Outbound ACL is not supported in Cisco ASR 900 RSP3 Module.

Information About Creating an IP Access List and Applying It to an Interface

Helpful Hints for Creating IP Access Lists

  • Create the access list before applying it to an interface (or elsewhere), because if you apply 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 immediate access problems.

  • Another reason to configure an access list before applying it is because an interface with an empty access list applied to it permits all traffic.

  • All access lists need at least one permit statement; otherwise, all packets are denied and no traffic passes.

  • Organize your access list so that more specific references in a network or subnet appear before more general ones.

  • A packet will match the first ACE in the ACL. Thus, a permit ip any any will match all packets, ignoring all subsequent ACES.

  • Although all access lists end with an implicit deny statement, we recommend use of an explicit deny statement (for example, deny ip any any). On most platforms, you can display the count of packets denied by issuing the show access-list command, thus finding out more information about who your access list is disallowing. Only packets denied by explicit deny statements are counted, which is why the explicit deny statement will yield more complete data for you.

  • While you are creating an access list or after it is created, you might want to delete an entry. You can delete an entry from a named access list. Use the no permit or no deny command to delete the appropriate entry.

  • In order to make the purpose of individual statements more scannable and easily understood at a glance, you can write a helpful remark before or after any statement by using the remark command.

  • If you want to deny access to a particular host or network and find out if someone from that network or host is attempting to gain access, include the log keyword with the corresponding deny statement so that the packets denied from that source are logged for you.

  • This hint applies to the placement of your access list. When trying to save resources, remember that an inbound access list applies the filter conditions before the routing table lookup. An outbound access list applies the filter conditions after the routing table lookup.

Access List Remarks

You can include comments or remarks about entries in any IP access list. An access list remark is an optional remark before or after an access list entry that describes the entry so that you do not have to interpret the purpose of the entry. Each remark is limited to 100 characters in length.

The remark can go before or after a permit or deny statement. Be consistent about where you add remarks. Users may be confused if some remarks precede the associated permit or deny statements and some remarks follow the associated statements.

The following is an example of a remark that describes function of the subsequent deny statement: 

ip access-list extended telnetting
 remark Do not allow host1 subnet to telnet out
 deny tcp host 172.16.2.88 any eq telnet

Additional IP Access List Features

Beyond the basic steps to create a standard or extended access list, you can enhance your access lists as mentioned below. Each of these methods is described completely in the Refining an IP Access List module.

  • You can impose dates and times when permit or deny statements in an extended access list are in effect, making your access list more granular and specific to an absolute or periodic time period.

  • After you create a named or numbered access list, you might want to add entries or change the order of the entries, known as resequencing an access list.

  • You can achieve finer granularity when filtering packets by filtering on noninitial fragments of packets.

How to Create an IP Access List and Apply It to an Interface

This section describes the general ways to create a standard or extended access list using either a name or a number. Access lists are very flexible; the tasks simply illustrate one permit command and one deny command to provide you the command syntax of each. Only you can determine how many permit and deny commands you need and their order.


Note

The first two tasks in this module create an access list; you must apply the access list in order for it to function. If you want to apply the access list to an interface, perform the task “Applying the Access List to an Interface”.

Creating a Standard Access List to Filter on Source Address

If you want to filter on source address only, a standard access list is simple and sufficient. There are two alternative types of standard access list: named and numbered. Named access lists allow you to identify your access lists with a more intuitive name rather than a number, and they also support more features than numbered access lists.

Creating a Named Access List to Filter on Source Address

Use a standard, named access list if you need to filter on source address only. This task illustrates one permit statement and one deny statement, but the actual statements you use and their order depend on what you want to filter or allow. Define your permit and deny statements in the order that achieves your filtering goals.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    ip access-list standard name

    4.    remark remark

    5.    deny {source [source-wildcard] | any} [log]

    6.    remark remark

    7.    permit {source [source-wildcard] | any} [log]

    8.    Repeat some combination of Steps 4 through 7 until you have specified the sources on which you want to base your access list.

    9.    end

    10.    show ip access-list


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example: 
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example: 
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 ip access-list standard name


    Example: 
    Device(config)# ip access-list standard R&D
     

    Defines a standard IP access list using a name and enters standard named access list configuration mode.

     
    Step 4 remark remark


    Example: 
    Device(config-std-nacl)# remark deny Sales network
     

    (Optional) Adds a user-friendly comment about an access list entry.

    • A remark can precede or follow an access list entry.

    • In this example, the remark reminds the network administrator that the subsequent entry denies the Sales network access to the interface (assuming this access list is later applied to an interface).

     
    Step 5deny {source [source-wildcard] | any} [log]

    Example: 
    Device(config-std-nacl)# deny 172.16.0.0 0.0.255.255 log
     

    (Optional) Denies the specified source based on a source address and wildcard mask.

    • If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

    • Optionally use the keyword any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

    • In this example, all hosts on network 172.16.0.0 are denied passing the access list.

    • Because this example explicitly denies a source address and the log keyword is specified, any packets from that source are logged when they are denied. This is a way to be notified that someone on a network or host is trying to gain access.

     
    Step 6 remark remark


    Example: 
    Device(config-std-nacl)# remark Give access to Tester’s host
     

    (Optional) Adds a user-friendly comment about an access list entry.

    • A remark can precede or follow an access list entry.

    • This remark reminds the network administrator that the subsequent entry allows the Tester’s host access to the interface.

     
    Step 7 permit {source [source-wildcard] | any} [log]


    Example: 
    Device(config-std-nacl)# permit 172.18.5.22 0.0.0.0
     

    Permits the specified source based on a source address and wildcard mask.

    • Every access list needs at least one permit statement; it need not be the first entry.

    • If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

    • Optionally use the keyword any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

    • In this example, host 172.18.5.22 is allowed to pass the access list.

     
    Step 8 Repeat some combination of Steps 4 through 7 until you have specified the sources on which you want to base your access list.  

    Remember that all sources not specifically permitted are denied by an implicit deny statement at the end of the access list.

     
    Step 9 end


    Example: 
    Device(config-std-nacl)# end
     

    Exits standard named access list configuration mode and enters privileged EXEC mode.

     
    Step 10 show ip access-list


    Example: 
    Device# show ip access-list
     

    (Optional) Displays the contents of all current IP access lists.

     

    Creating a Numbered Access List to Filter on Source Address

    Configure a standard, numbered access list if you need to filter on source address only and you prefer not to use a named access list.

    IP standard access lists are numbered 1 to 99 or 1300 to 1999. This task illustrates one permit statement and one deny statement, but the actual statements you use and their order depend on what you want to filter or allow. Define your permit and deny statements in the order that achieves your filtering goals.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    access-list access-list-number permit {source [source-wildcard] | any} [log]

      4.    access-list access-list-number deny {source [source-wildcard] | any} [log]

      5.    Repeat some combination of Steps 3 through 6 until you have specified the sources on which you want to base your access list.

      6.    end

      7.    show ip access-list


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example: 
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 configure terminal


      Example: 
      Device# configure terminal
       

      Enters global configuration mode.

       
      Step 3 access-list access-list-number permit {source [source-wildcard] | any} [log]


      Example: 
      Device(config)# access-list 1 permit 172.16.5.22 0.0.0.0
       

      Permits the specified source based on a source address and wildcard mask.

      • Every access list needs at least one permit statement; it need not be the first entry.

      • Standard IP access lists are numbered 1 to 99 or 1300 to 1999.

      • If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

      • Optionally use the keyword any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

      • In this example, host 172.16.5.22 is allowed to pass the access list.

       
      Step 4 access-list access-list-number deny {source [source-wildcard] | any} [log]


      Example: 
      Device(config)# access-list 1 deny 172.16.7.34 0.0.0.0
       

      Denies the specified source based on a source address and wildcard mask.

      • If the source-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source address.

      • Optionally use the abbreviation any as a substitute for the source source-wildcard to specify the source and source wildcard of 0.0.0.0 255.255.255.255.

      • In this example, host 172.16.7.34 is denied passing the access list.

       
      Step 5 Repeat some combination of Steps 3 through 6 until you have specified the sources on which you want to base your access list.  

      Remember that all sources not specifically permitted are denied by an implicit deny statement at the end of the access list.

       
      Step 6 end


      Example: 
      Device(config)# end
       

      Exits global configuration mode and enters privileged EXEC mode.

       
      Step 7 show ip access-list


      Example: 
      Device# show ip access-list
       

      (Optional) Displays the contents of all current IP access lists.

       

      Creating an Extended Access List

      If you want to filter on anything other than source address, you need to create an extended access list. There are two alternative types of extended access list: named and numbered. Named access lists allow you to identify your access lists with a more intuitive name rather than a number, and they also support more features.

      For details on how to filter something other than source or destination address, see the syntax descriptions in the command reference documentation.

      Creating a Named Extended Access List

      Create a named extended access list if you want to filter the source and destination address or filter a combination of addresses and other IP fields.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    ip access-list extended name

        4.    deny protocol source [source-wildcard] destination [destination-wildcard] [option option-name] [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]

        5.    permit protocol source [source-wildcard] destination [destination-wildcard] [option option-name] [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]

        6.    Repeat some combination of Steps 4 through 7 until you have specified the fields and values on which you want to base your access list.

        7.    end

        8.    show ip access-list


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example: 
        Device> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.

         
        Step 2 configure terminal


        Example: 
        Device# configure terminal
         

        Enters global configuration mode.

         
        Step 3 ip access-list extended name


        Example: 
        Device(config)# ip access-list extended acl1
         

        Defines an extended IP access list using a name and enters extended named access list configuration mode.

         
        Step 4 deny protocol source [source-wildcard] destination [destination-wildcard] [option option-name] [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]


        Example: 
        Device(config-ext-nacl)# deny ip 172.18.0.0 0.0.255.255 host 172.16.40.10 log
         

        (Optional) Denies any packet that matches all of the conditions specified in the statement.

        • If the source-wildcard or destination-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source or destination address, respectively.

        • Optionally use the keyword any as a substitute for the source source-wildcard or destination destination-wildcard to specify the address and wildcard of 0.0.0.0 255.255.255.255.

        • Optionally use the keyword host source to indicate a source and source wildcard of source 0.0.0.0 or the abbreviation host destination to indicate a destination and destination wildcard of destination 0.0.0.0.

        • In this example, packets from all sources are denied access to the destination network 172.18.0.0. Logging messages about packets permitted or denied by the access list are sent to the facility configured by the logging facility command (for example, console, terminal, or syslog). That is, any packet that matches the access list will cause an informational logging message about the packet to be sent to the configured facility. The level of messages logged to the console is controlled by the logging console command.

         
        Step 5 permit protocol source [source-wildcard] destination [destination-wildcard] [option option-name] [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]


        Example: 
        Device(config-ext-nacl)# permit tcp any any
         

        Permits any packet that matches all of the conditions specified in the statement.

        • Every access list needs at least one permit statement.

        • If the source-wildcard or destination-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source or destination address, respectively.

        • Optionally use the keyword any as a substitute for the source source-wildcard or destination destination-wildcard to specify the address and wildcard of 0.0.0.0 255.255.255.255.

        • In this example, TCP packets are allowed from any source to any destination.

        • Use the log-input keyword to include input interface, source MAC address, or virtual circuit in the logging output.

         
        Step 6 Repeat some combination of Steps 4 through 7 until you have specified the fields and values on which you want to base your access list.  

        Remember that all sources not specifically permitted are denied by an implicit deny statement at the end of the access list.

         
        Step 7 end


        Example: 
        Device(config-ext-nacl)# end
         

        Exits standard named access list configuration mode and enters privileged EXEC mode.

         
        Step 8 show ip access-list


        Example: 
        Device# show ip access-list
         

        (Optional) Displays the contents of all current IP access lists.

         
        RSP3 Porting Related Information

        ACL is not supported for fragmented packets.

        Creating a Numbered Extended Access List

        Create a numbered extended access list if you want to filter on source and destination address, or a combination of addresses and other IP fields, and you prefer not to use a name. Extended IP access lists are numbered 100 to 199 or 2000 to 2699.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    access-list access-list-number remark remark

          4.    access-list access-list-number permit protocol {source [source-wildcard] | any} {destination [destination-wildcard] | any} [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]

          5.    access-list access-list-number remark remark

          6.    access-list access-list-number deny protocol {source [source-wildcard] | any} {destination [destination-wildcard] | any} [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]

          7.    Repeat some combination of Steps 3 through 6 until you have specified the fields and values on which you want to base your access list.

          8.    end

          9.    show ip access-list


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example: 
          Device> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.

           
          Step 2 configure terminal


          Example: 
          Device# configure terminal
           

          Enters global configuration mode.

           
          Step 3 access-list access-list-number remark remark


          Example: 
          Device(config)# access-list 107 remark allow Telnet packets from any source to network 172.69.0.0 (headquarters)
           

          (Optional) Adds a user-friendly comment about an access list entry.

          • A remark of up to 100 characters can precede or follow an access list entry.

           
          Step 4 access-list access-list-number permit protocol {source [source-wildcard] | any} {destination [destination-wildcard] | any} [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]


          Example: 
          Device(config)# access-list 107 permit tcp any 172.69.0.0 0.0.255.255 eq telnet
           

          Permits any packet that matches all of the conditions specified in the statement.

          • Every access list needs at least one permit statement; it need not be the first entry.

          • Extended IP access lists are numbered 100 to 199 or 2000 to 2699.

          • If the source-wildcard or destination-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source or destination address, respectively.

          • Optionally use the keyword any as a substitute for the source source-wildcard or destination destination-wildcard to specify the address and wildcard of 0.0.0.0 255.255.255.255.

          • TCP and other protocols have additional syntax available. See the access-list command in the command reference for complete syntax.

           
          Step 5 access-list access-list-number remark remark


          Example: 
          Device(config)# access-list 107 remark deny all other TCP packets
           

          (Optional) Adds a user-friendly comment about an access list entry.

          • A remark of up to 100 characters can precede or follow an access list entry.

           
          Step 6 access-list access-list-number deny protocol {source [source-wildcard] | any} {destination [destination-wildcard] | any} [precedence precedence] [tos tos] [established] [log | log-input] [time-range time-range-name] [fragments]


          Example: 
          Device(config)# access-list 107 deny tcp any any
           

          Denies any packet that matches all of the conditions specified in the statement.

          • If the source-wildcard or destination-wildcard is omitted, a wildcard mask of 0.0.0.0 is assumed, meaning match on all bits of the source or destination address, respectively.

          • Optionally use the keyword any as a substitute for the source source-wildcard or destination destination-wildcard to specify the address and wildcard of 0.0.0.0 255.255.255.255.

           
          Step 7 Repeat some combination of Steps 3 through 6 until you have specified the fields and values on which you want to base your access list.  

          Remember that all sources not specifically permitted are denied by an implicit deny statement at the end of the access list.

           
          Step 8 end


          Example: 
          Device(config)# end
           

          Exits global configuration mode and enters privileged EXEC mode.

           
          Step 9 show ip access-list


          Example: 
          Device# show ip access-list
           

          (Optional) Displays the contents of all current IP access lists.

           

          Applying an Access List to an Interface

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    interface type number

            4.    ip access-group {access-list-number | access-list-name} {in | out}

            5.    end


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 enable


            Example: 
            Device> enable
             

            Enables privileged EXEC mode.

            • Enter your password if prompted.

             
            Step 2 configure terminal


            Example: 
            Device# configure terminal
             

            Enters global configuration mode.

             
            Step 3 interface type number


            Example:

             

            Specifies an interface and enters interface configuration mode.

             
            Step 4 ip access-group {access-list-number | access-list-name} {in | out}


            Example: 
            Device(config-if)# ip access-group acl1 in
             

            Applies the specified access list to the inbound interface.

            • To filter source addresses, apply the access list to the inbound interface.

             
            Step 5 end


            Example: 
            Device(config-if)# end
             

            Exits interface configuration mode and returns to privileged EXEC mode.

             

            Configuration Examples for Creating an IP Access List and Applying It to an Interface

            Example: Filtering on Host Source Address

            In the following example, the workstation belonging to user1 is allowed access to gigabitethernet 0/0/0, and the workstation belonging to user2 is not allowed access: 

            interface gigabitethernet 0/0/0
 ip access-group workstations in
!
ip access-list standard workstations
 remark Permit only user1 workstation through
 permit 172.16.2.88
 remark Do not allow user2 workstation through
 deny 172.16.3.13

            Example: Filtering on Subnet Source Address

            In the following example, the user1 subnet is not allowed access to gigabitethernet interface 0/0/0, but the Main subnet is allowed access: 

            interface gigabitethernet 0/0/0
 ip access-group prevention in
!
ip access-list standard prevention
 remark Do not allow user1 subnet through
 deny 172.22.0.0 0.0.255.255
 remark Allow Main subnet
 permit 172.25.0.0 0.0.255.255

            Example: Filtering on Source and Destination Addresses and IP Protocols

            The following configuration example shows an interface with two access lists, one applied to outgoing packets and one applied to incoming packets. The standard access list named Internet-filter filters outgoing packets on source address. The only packets allowed out the interface must be from source 172.16.3.4.

            The extended access list named marketing-group filters incoming packets. The access list permits Telnet packets from any source to network 172.26.0.0 and denies all other TCP packets. It permits any ICMP packets. It denies UDP packets from any source to network 172.26.0 0 on port numbers less than 1024. Finally, the access list denies all other IP packets and performs logging of packets passed or denied by that entry. 

            interface gigabitethernet 0/0/0
 ip address 172.20.5.1 255.255.255.0
 ip access-group Internet-filter out
 ip access-group marketing-group in
!
ip access-list standard Internet-filter
 permit 172.16.3.4
ip access-list extended marketing-group
 permit tcp any 172.26.0.0 0.0.255.255 eq telnet
 deny tcp any any
 permit icmp any any
 deny udp any 172.26.0.0 0.0.255.255 lt 1024
 deny ip any any

            Example: Filtering on Source Addresses Using a Numbered Access List

            In the following example, network 10.0.0.0 is a Class A network whose second octet specifies a subnet; that is, its subnet mask is 255.255.0.0. The third and fourth octets of a network 10.0.0.0 address specify a particular host. Using access list 2, the Cisco IOS XE software would accept one address on subnet 48 and reject all others on that subnet. The last line of the list shows that the software would accept addresses on all other network 10.0.0.0 subnets. 

            interface gigabitethernet 0/0/0
 ip access-group 2 in
!
access-list 2 permit 10.48.0.3
access-list 2 deny 10.48.0.0  0.0.255.255 
access-list 2 permit 10.0.0.0  0.255.255.255

            Example: Preventing Telnet Access to a Subnet

            In the following example, the user1 subnet is not allowed to telnet out of gigabitethernet interface 0/0/0: 

            interface gigabitethernet 0/0/0
 ip access-group telnetting out
!
ip access-list extended telnetting
 remark Do not allow user1 subnet to telnet out
 deny tcp 172.20.0.0 0.0.255.255 any eq telnet
 remark Allow Top subnet to telnet out
 permit tcp 172.33.0.0 0.0.255.255 any eq telnet

            Example: Filtering on TCP and ICMP Using Port Numbers

            In the following example, the first line of the extended access list named acl1 permits any incoming TCP connections with destination ports greater than 1023. The second line permits incoming TCP connections to the Simple Mail Transfer Protocol (SMTP) port of host 172.28.1.2. The last line permits incoming ICMP messages for error feedback. 

            interface gigabitethernet 0/0/0
 ip access-group acl1 in
!
ip access-list extended acl1
 permit tcp any 172.28.0.0 0.0.255.255 gt 1023
 permit tcp any host 172.28.1.2 eq 25
 permit icmp any 172.28.0.0 255.255.255.255

            Example: Allowing SMTP E-mail and Established TCP Connections

            Suppose you have a network connected to the Internet, and you want any host on an Ethernet to be able to form TCP connections to any host on the Internet. However, you do not want IP hosts to be able to form TCP connections to hosts on the gigabitethernet except to the mail (SMTP) port of a dedicated mail host.

            SMTP uses TCP port 25 on one end of the connection and a random port number on the other end. The same two port numbers are used throughout the life of the connection. Mail packets coming in from the Internet will have a destination port of 25. Outbound packets will have the port numbers reversed. The fact that the secure system behind the router always will accept mail connections on port 25 is what makes possible separate control of incoming and outgoing services. The access list can be configured on either the outbound or inbound interface.

            In the following example, the gigabitethernet network is a Class B network with the address 172.18.0.0, and the address of the mail host is 172.18.1.2. The established keyword is used only for the TCP protocol to indicate an established connection. A match occurs if the TCP datagram has the ACK or RST bits set, which indicate that the packet belongs to an existing connection. 

            interface gigabitethernet 0/0/0
 ip access-group 102 in
!
access-list 102 permit tcp any 172.18.0.0 0.0.255.255 established
access-list 102 permit tcp any host 172.18.1.2 eq 25

            Example: Preventing Access to the Web by Filtering on Port Name

            In the following example, the w1and w2 workstations are not allowed web access; other hosts on network 172.20.0.0 are allowed web access: 

            interface gigabitethernet0/0/0
 ip access-group no-web out
!
ip access-list extended no-web
 remark Do not allow w1 to browse the web
 deny host 172.20.3.85 any eq http
 remark Do not allow w2 to browse the web
 deny host 172.20.3.13 any eq http
 remark Allow others on our network to browse the web
 permit 172.20.0.0 0.0.255.255 any eq http

            Example: Filtering on Source Address and Logging the Packets

            The following example defines access lists 1 and 2, both of which have logging enabled: 

            interface gigabitethernet 0/0/0
 ip address 172.16.1.1 255.0.0.0
 ip access-group 1 in
 
!
access-list 1 permit 172.25.0.0 0.0.255.255 log
access-list 1 deny 172.30.0.0 0.0.255.255 log
!
access-list 2 permit 172.27.3.4 log
access-list 2 deny 172.17.0.0 0.0.255.255 log

            If the interface receives 10 packets from 172.25.7.7 and 14 packets from 172.17.23.21, the first log will look like the following: 

            list 1 permit 172.25.7.7 1 packet
list 2 deny 172.17.23.21 1 packet

            Five minutes later, the console will receive the following log: 

            list 1 permit 172.25.7.7 9 packets
list 2 deny 172.17.23.21 13 packets

            Example: Limiting Debug Output

            The following sample configuration uses an access list to limit the debug command output. Limiting the debug output restricts the volume of data to what you are interested in, saving you time and resources. 

            Device(config)# ip access-list acl1
Device(config-std-nacl)# remark Displays only advertisements for LDP peer in acl1
Device(config-std-nacl)# permit host 10.0.0.44

Device# debug mpls ldp advertisements peer-acl acl1

tagcon: peer 10.0.0.44:0 (pp 0x60E105BC): advertise 172.17.0.33
tagcon: peer 10.0.0.44:0 (pp 0x60E105BC): advertise 172.16.0.31
tagcon: peer 10.0.0.44:0 (pp 0x60E105BC): advertise 172.22.0.33
tagcon: peer 10.0.0.44:0 (pp 0x60E105BC): advertise 192.168.0.1
tagcon: peer 10.0.0.44:0 (pp 0x60E105BC): advertise 192.168.0.3
tagcon: peer 10.0.0.44:0 (pp 0x60E105BC): advertise 192.168.1.33

            Additional References Creating an IP Access List and Applying It to an Interface

            Related Documents

            Related Topic

            Document Title

            Cisco IOS commands

            Cisco IOS Master Command List, All Releases

            Security commands

            • Order of access list entries

            • Access list entries based on time of day or week

            • Packets with noninitial fragments

            Refining an IP Access List

            Filtering on IP options, TCP flags, or noncontiguous ports

            Creating an IP Access List for Filtering

            Controlling logging-related parameters

            Understanding Access Control List Loggingl

            Standards and RFCs

            Standard/RFC

            Title

            No new or modified standards or RFCs are supported by this feature, and support for existing standards or RFCs has not been modified by this feature.

            —

            Technical Assistance

            Description

            Link

            The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

            http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

            Feature Information for Creating an IP Access List and Applying It to an Interface

            The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

            Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.
            Table 1 Feature Information for Creating IP Access Lists and Applying It to an Interface

            Feature Name

            Releases

            Feature Configuration Information

            ACL—Access Control List Source and Destination Address Matching

            Cisco IOS XE Release 3.5S

            In the Cisco IOS XE Release 3.5S, support was added for the Cisco ASR 903 Router.

            ACL—ICMP Code

            Cisco IOS XE Release 3.5S

            In the Cisco IOS XE Release 3.5S, support was added for the Cisco ASR 903 Router.

            ACL Performance Enhancement

            Cisco IOS XE Release 2.1

            This feature was introduced on the Cisco ASR 1000 Series Aggregation Services Routers.

            No commands were introduced or modified for this feature.