5.4.12 Packet Tracer – Configure Extended IPv4 ACLs – Scenario 1 Answers

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Answers Note: Red font color or gray highlights indicate text that appears in the instructor copy only.

Addressing Table

Objectives

Part 1: Configure, Apply and Verify an Extended Numbered ACL

Part 2: Configure, Apply and Verify an Extended Named ACL

Background / Scenario

Two employees need access to services provided by the server. PC1 only needs FTP access while PC2 only needs web access. Both computers need to be able to ping the server, but not each other.

Instructions

Part 1:  Configure, Apply and Verify an Extended Numbered ACL

Step 1:  Configure an ACL to permit FTP and ICMP from PC1 LAN.

1. From global configuration mode on R1, enter the following command to determine the first valid number for an extended access list.

Open configuration window

R1(config)# access-list ?

  <1-99>     IP standard access list

  <100-199>  IP extended access list

2. Add 100 to the command, followed by a question mark.

R1(config)# access-list 100 ?

  deny    Specify packets to reject

  permit  Specify packets to forward

  remark  Access list entry comment

3. To permit FTP traffic, enter permit, followed by a question mark.

R1(config)# access-list 100 permit ?

  ahp    Authentication Header Protocol

  eigrp  Cisco’s EIGRP routing protocol

  esp    Encapsulation Security Payload

  gre    Cisco’s GRE tunneling

  icmp   Internet Control Message Protocol

  ip     Any Internet Protocol

  ospf   OSPF routing protocol

  tcp    Transmission Control Protocol

  udp    User Datagram Protocol

4. When configured and applied, this ACL should permit FTP and ICMP. ICMP is listed above, but FTP is not. This is because FTP is an application layer protocol that uses TCP at the transport layer. Enter TCP to further refine the ACL help.

R1(config)# access-list 100 permit tcp ?

  A.B.C.D  Source address

  any      Any source host

  host     A single source host

5. The source address can represent a single device, such as PC1, by using the host keyword and then the IP address of PC1. Using the keyword any permits any host on any network. Filtering can also be done by a network address. In this case, it is any host that has an address belonging to the 172.22.34.64/27 network. Enter this network address, followed by a question mark.

R1(config)# access-list 100 permit tcp 172.22.34.64 ?

  A.B.C.D  Source wildcard bits

6. Calculate the wildcard mask by determining the binary opposite of the /27 subnet mask.

11111111.11111111.11111111.11100000 = 255.255.255.224

00000000.00000000.00000000.00011111 = 0.0.0.31

7. Enter the wildcard mask, followed by a question mark.

R1(config)# access-list 100 permit tcp 172.22.34.64 0.0.0.31 ?

  A.B.C.D  Destination address

  any      Any destination host

  eq       Match only packets on a given port number

  gt       Match only packets with a greater port number

  host     A single destination host

  lt       Match only packets with a lower port number

  neq      Match only packets not on a given port number

  range    Match only packets in the range of port numbers

8. Configure the destination address. In this scenario, we are filtering traffic for a single destination, which is the server. Enter the host keyword followed by the server’s IP address.

R1(config)# access-list 100 permit tcp 172.22.34.64 0.0.0.31 host 172.22.34.62 ?

  dscp         Match packets with given dscp value

  eq           Match only packets on a given port number

  established  established

  gt           Match only packets with a greater port number

  lt           Match only packets with a lower port number

  neq          Match only packets not on a given port number

  precedence   Match packets with given precedence value

  range        Match only packets in the range of port numbers

  <cr>

9. Notice that one of the options is <cr> (carriage return). In other words, you can press Enter and the statement would permit all TCP traffic. However, we are only permitting FTP traffic; therefore, enter the eq keyword, followed by a question mark to display the available options. Then, enter ftp and press Enter.

R1(config)# access-list 100 permit tcp 172.22.34.64 0.0.0.31 host 172.22.34.62 eq ?

  <0-65535>  Port number

  ftp        File Transfer Protocol (21)

  pop3       Post Office Protocol v3 (110)

  smtp       Simple Mail Transport Protocol (25)

  telnet     Telnet (23)

  www        World Wide Web (HTTP, 80)

R1(config)# access-list 100 permit tcp 172.22.34.64 0.0.0.31 host 172.22.34.62 eq ftp

10. Create a second access list statement to permit ICMP (ping, etc.) traffic from PC1 to Server. Note that the access list number remains the same and a specific type of ICMP traffic does not need to be specified.

R1(config)# access-list 100 permit icmp 172.22.34.64 0.0.0.31 host 172.22.34.62

11. All other traffic is denied, by default.
12. Execute the show access-list command and verify that access list 100 contains the correct statements. Notice that the statement deny any any does not appear at the end of the access list. The default execution of an access list is that if a packet does not match a statement in the access list, it is not permitted through the interface.

R1#show access-lists

Extended IP access list 100

    10 permit tcp 172.22.34.64 0.0.0.31 host 172.22.34.62 eq ftp

    20 permit icmp 172.22.34.64 0.0.0.31 host 172.22.34.62

Step 2:  Apply the ACL on the correct interface to filter traffic.

From R1’s perspective, the traffic that ACL 100 applies to is inbound from the network connected to the Gigabit Ethernet 0/0 interface. Enter interface configuration mode and apply the ACL.

Note: On an actual operational network, it is not a good practice to apply an untested access list to an active interface.

R1(config)# interface gigabitEthernet 0/0

R1(config-if)# ip access-group 100 in

Step 3:  Verify the ACL implementation.

1. Ping from PC1 to Server. If the pings are unsuccessful, verify the IP addresses before continuing.
2. FTP from PC1 to Server. The username and password are both cisco.

PC> ftp 172.22.34.62

3. Exit the FTP service.

ftp> quit

close configuration window

4. Ping from PC1 to PC2. The destination host should be unreachable, because the ACL did not explicitly permit the traffic.

Part 2:  Configure, Apply and Verify an Extended Named ACL

Step 1:  Configure an ACL to permit HTTP access and ICMP from PC2 LAN.

1. Named ACLs start with the ip keyword. From global configuration mode of R1, enter the following command, followed by a question mark.

Open configuration window

R1(config)# ip access-list ?

  extended  Extended Access List

  standard  Standard Access List

2. You can configure named standard and extended ACLs. This access list filters both source and destination IP addresses; therefore, it must be extended. Enter HTTP_ONLY as the name. (For Packet Tracer scoring, the name is case-sensitive and the access list statements must be the correct order.)

R1(config)# ip access-list extended HTTP_ONLY

3. The prompt changes. You are now in extended named ACL configuration mode. All devices on the PC2 LAN need TCP access. Enter the network address, followed by a question mark.

R1(config-ext-nacl)# permit tcp 172.22.34.96 ?

  A.B.C.D  Source wildcard bits

4. An alternative way to calculate a wildcard is to subtract the subnet mask from 255.255.255.255.

  255.255.255.255

– 255.255.255.240

—————–

=   0.  0.  0. 15

R1(config-ext–nacl)# permit tcp 172.22.34.96 0.0.0.15

5. Finish the statement by specifying the server address as you did in Part 1 and filtering www traffic.

R1(config-ext–nacl)# permit tcp 172.22.34.96 0.0.0.15 host 172.22.34.62 eq www

6. Create a second access list statement to permit ICMP (ping, etc.) traffic from PC2 to Server. Note: The prompt remains the same and a specific type of ICMP traffic does not need to be specified.

R1(config-ext-nacl)# permit icmp 172.22.34.96 0.0.0.15 host 172.22.34.62

7. All other traffic is denied, by default. Exit extended named ACL configuration mode.
8. Execute the show access-list command and verify that access list HTTP_ONLY contains the correct statements.

R1# show access-lists

Extended IP access list 100

10 permit tcp 172.22.34.64 0.0.0.31 host 172.22.34.62 eq ftp

20 permit icmp 172.22.34.64 0.0.0.31 host 172.22.34.62

Extended IP access list HTTP_ONLY

10 permit tcp 172.22.34.96 0.0.0.15 host 172.22.34.62 eq www

20 permit icmp 172.22.34.96 0.0.0.15 host 172.22.34.62

Step 2:  Apply the ACL on the correct interface to filter traffic.

From R1’s perspective, the traffic that access list HTTP_ONLY applies to is inbound from the network connected to the Gigabit Ethernet 0/1 interface. Enter interface configuration mode and apply the ACL.

Note: On an actual operational network, it is not a good practice to apply an untested access list to an active interface.  It should be avoided if possible.

R1(config)# interface gigabitEthernet 0/1

R1(config-if)# ip access-group HTTP_ONLY in

Step 3:  Verify the ACL implementation.

1. Ping from PC2 to Server. If the ping is unsuccessful, verify the IP addresses before continuing.
2. From PC2 open a web browser and enter the IP address of the Server. The web page of the Server should be displayed.
3. FTP from PC2 to Server. The connection should fail. If not, troubleshoot the access list statements and the access-group configurations on the interfaces.

Close configuration window

End of document

Answer Script

Router R1

enable

configure terminal

access-list 100 permit tcp 172.22.34.64 0.0.0.31 host 172.22.34.62 eq ftp

access-list 100 permit icmp 172.22.34.64 0.0.0.31 host 172.22.34.62

interface gigabitEthernet 0/0

 ip access-group 100 in

ip access-list extended HTTP_ONLY

 permit tcp 172.22.34.96 0.0.0.15

 permit tcp 172.22.34.96 0.0.0.15 host 172.22.34.62 eq www

 permit icmp 172.22.34.96 0.0.0.15 host 172.22.34.62

interface gigabitEthernet 0/1

ip access-group HTTP_ONLY in

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