Extended ACL

1. What is an Extended ACL

When managing a network, sometimes you need more than just blocking traffic based on the source IP address.

That’s where Extended Access Control Lists (ACLs) come in.

Unlike Standard ACLs, an Extended ACL lets you filter traffic based on source IP, destination IP, protocol, and port numbers.

Let’s say you have this topology:

Figure 1 – Extended ACL Use Case

In this example, we want to control access using an Extended ACL:

• Allow HTTPS traffic from the Legal PC to a secure Legal Server at 192.168.2.1

• Deny all access from the Legal PC to a HR Server at 192.168.3.1

This is a typical use case where Extended ACLs are necessary because Standard ACLs cannot filter by protocol or port.

2. Configure Extended ACL

Now that you understand what an Extended ACL is, let’s walk through how to configure one step by step on a Cisco router.

Step 1 - Enter Global Configuration Mode

Start by entering configuration mode:

R1# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#

Then, check available ACL types with the ip access-list ? command:

R1(config)# ip access-list ?
  extended    Extended Access List
  helper      Access List acts on helper-address
  log-update  Control access list log updates
  logging     Control access list logging
  resequence  Resequence Access List
  standard    Standard Access List

As you can see, we have the option to create either a standard or an extended ACL.

We’ll proceed with extended.

Step 2 – Create the Extended ACL

Now let's check the available formats for Extended ACLs:

R1(config)# ip access-list extended ?
  <100-199>    Extended IP access-list number
  <2000-2699>  Extended IP access-list number (expanded range)
  WORD         Access-list name

Extended ACLs can be:

• Numbered (ranges: 100–199 or 2000–2699)

• Named (custom name of your choice)

Let’s create a numbered ACL 101:

In this example, we’ll create a numbered Extended ACL with the number 101.

R1(config)# ip access-list extended 101
R1(config-ext-nacl)#

You are now in Extended ACL configuration mode, ready to define rules.

Step 3 - Permit HTTPS to Legal Server

We want to allow HTTPS traffic (TCP port 443) from the Legal PC to the Legal Server.

🔹 Choose the Protocol

Start with permit, then see the protocol options:

R1(config-ext-nacl)# permit ?
  <0-255>       An IP protocol number
  ahp           Authentication Header Protocol
  eigrp         Cisco's EIGRP routing protocol
  esp           Encapsulation Security Payload
  gre           Cisco's GRE tunneling
  icmp          Internet Control Message Protocol
  igmp          Internet Gateway Message Protocol
  ip            Any Internet Protocol
  ipinip        IP in IP tunneling
  nos           KA9Q NOS compatible IP over IP tunneling
  object-group  Service object group
  ospf          OSPF routing protocol
  pcp           Payload Compression Protocol
  pim           Protocol Independent Multicast
  sctp          Stream Control Transmission Protocol
  tcp           Transmission Control Protocol
  udp           User Datagram Protocol

We want HTTPS, which runs over TCP, so:

R1(config-ext-nacl)# permit tcp ?
  A.B.C.D       Source address
  any           Any source host
  host          A single source host
  object-group  Source network object group

🔹 Define the Source IP Address

Let's specify the Legal PC as the source:

R1(config-ext-nacl)# permit tcp host 192.168.1.1 ?
  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
  object-group  Destination network object group
  range         Match only packets in the range of port numbers

🔹 Define the Destination IP Address

We want to reach the Legal Web Server:

R1(config-ext-nacl)# permit tcp host 192.168.1.1 host 192.168.2.1 ?
  ack          Match on the ACK bit
  dscp         Match packets with given dscp value
  eq           Match only packets on a given port number
  established  Match established connections
  fin          Match on the FIN bit
  fragments    Check non-initial fragments
// OUTPUT OMITTED FOR BREVITY

In order to choose the port number to filter, we use eq as you can see in the cli.

🔹 Match the HTTPS Port (443)

We now filter only HTTPS traffic (port 443)

R1(config-ext-nacl)# permit tcp host 192.168.1.1 host 192.168.2.1 eq ?
  <0-65535>    Port number
  bgp          Border Gateway Protocol (179)
  chargen      Character generator (19)
  cmd          Remote commands (rcmd, 514)
  daytime      Daytime (13)
  discard      Discard (9)
  domain       Domain Name Service (53)
  drip         Dynamic Routing Information Protocol (3949)
  echo         Echo (7)
  exec         Exec (rsh, 512)
  finger       Finger (79)
  ftp          File Transfer Protocol (21)
// OUTPUT OMITTED FOR BREVITY

And we complete the command with the port:

R1(config-ext-nacl)# permit tcp host 192.168.1.1 host 192.168.2.1 eq 443

This command allows only HTTPS traffic from the Legal PC to the Legal Server.

Step 4 - Deny All Traffic to HR Server

Every ACL ends with an implicit deny all.
So any traffic that isn’t explicitly allowed will automatically be blocked.

In our case:

• We allowed HTTPS traffic to the Legal Server (192.168.2.1)

• We did not allow anything to the HR Server (192.168.3.1)

→ This traffic is already denied by default.

But just for demonstration, here’s how to create a deny rule manually, this is to show you how it's done:

R1(config-ext-nacl)# deny ip host 192.168.1.1 host 192.168.3.1

Table 1 – Breakdown of an Explicit Deny Rule in an Extended ACL

This rule is not required in this case it’s here to show you how to write an explicit deny rule.

3. Where to Apply Extended ACL

An Extended ACL won’t filter any traffic until it’s applied to an interface.
This step tells the router where to inspect traffic and in which direction.

Placement Strategy

Extended ACLs filter traffic based on:

• Source IP

• Destination IP

• Protocol

• Port number

To reduce unnecessary traffic across the network, Extended ACLs should be applied as close as possible to the source.

This way, unwanted packets are dropped early before consuming bandwidth.

Exemple Scenario

In our example:

• The Legal PC (192.168.1.1) is the source

• We want to:

  • Allow HTTPS traffic to the Legal Server (192.168.2.1)

  • Block all access to the HR Server (192.168.3.1)

Figure 2 – Extended ACL Placement

The traffic enters router R1 through interface G0/0 which is connected to the Legal PC.

That’s interface G0/0 on router R1.

Applying the ACL to Interface

🔹 Step 1 – Enter interface configuration mode

R1(config)# int g0/0

🔹 Step 2 – Check available formats

Use the ip access-group command:

R1(config-if)# ip access-group ?
  <1-199>      IP access list (standard or extended)
  <1300-2699>  IP expanded access list (standard or extended)
  WORD         Access-list name

We'll use the ACL 101 that we created earlier.

🔹 Step 3 – Apply the ACL inbound

We apply ACL 101 to inspect packets coming from the source:

R1(config-if)# ip access-group 101 ?
  in   inbound packets
  out  outbound packets
R1(config-if)# ip access-group 101 in

This command activates ACL 101 on interface G0/0.
From now on, all inbound packets will be filtered according to your rules.

4. Verifying Extended ACLs

Once your ACL is applied, it's essential to confirm that:

• The ACL contains the correct rules

• It is properly applied to the correct interface and in the right direction

Step 1 – View the ACL Configuration

R1# show access-lists 101
Extended IP access list 101
    10 permit tcp host 192.168.1.1 host 192.168.2.1 eq 443
    20 deny   ip host 192.168.1.1 host 192.168.3.1

This confirms:

• Line 10: Permits HTTPS traffic from 192.168.1.1 to 192.168.2.1

• Line 20: Denies all traffic from 192.168.1.1 to 192.168.3.1

Step 2 – Confirm Interface Association

To verify that the ACL is correctly applied to an interface (in this case, G0/0):

R1# show ip interface g0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet address is 192.168.1.254/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.10
  Outgoing access list is not set
  Inbound  access list is 101
  Proxy ARP is enabled
// OUTPUT OMITTED FOR BREVITY

This confirms that:

• ACL 101 is applied to GigabitEthernet0/0

• It is used in the inbound direction, as expected

5. Conclusion

Here’s what you need to remember about Standard vs Extended ACLs before moving on:

Table 2 – Standard vs Extended ACLs

💡 Do you want to use names instead of numbers for your ACLs?

Check out the next lesson on Named ACLs to make your configurations easier to manage.