Standard ACL

When managing a network, you often need to control which traffic is allowed or denied.
A Standard Access Control List (ACL) filters IPv4 traffic solely based on the source IP address.

Figure 1 – Standard ACL Use Case

In the example below:

• The Legal network (192.168.1.0/24) is allowed to access the Legal Server (192.168.3.1).

• The HR network (192.168.2.0/24) is denied access to that same server.

This is a perfect use case for a Standard ACL, you only care about who is sending the traffic, not where it's going or what kind of traffic it is.

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

Step 1 - Enter global configuration mode

R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.

Step 2 - Create the Standard ACL

To create a Standard ACL, use the ip access-list standard command.

R1(config)# ip access-list standard ? 
  <1-99>        Standard IP access-list number
  <1300-1999>   Standard IP access-list number (expanded range)
  WORD          Access-list name

Here we choose the number 10, which is part of the valid ranges for Standard ACLs:

• From 1 to 99 (classic range)

• From 1300 to 1999 (expanded range)

R1(config)# ip access-list standard 10 

Step 3 - Permit the Legal network

We want to allow traffic from the 192.168.1.0/24 network.

Let's see the available options:

R1(config-std-nacl)# permit ?
  Hostname or A.B.C.D  Address to match
  any                  Any source host
  host                 A single host address

Here we choose to match a network address:

R1(config-std-nacl)# permit 192.168.1.0 ?      
  A.B.C.D  Wildcard bits
  log      Log matches against this entry

We'll match a the network using a wildcard mask:

R1(config-std-nacl)# permit 192.168.1.0 0.0.0.255

This line matches any host from 192.168.1.0 to 192.168.1.255.

Why 0.0.0.255?

In ACLs, we don't use subnet masks, we use wildcard masks.

A wildcard mask is the inverse of a subnet mask. It tells the router which bits to ignore when comparing IP addresses.

R1(config-std-nacl)# permit 192.168.1.0 0.0.0.255

Table 1 – Subnet Mask vs Wildcard Mask in ACLs

So:

permit 192.168.1.0 0.0.0.255 => allows the whole 192.168.1.0/24 network
permit 192.168.1.10 0.0.0.0 => allows only 192.168.1.10

Step 4 - Deny the HR network

Now we block all traffic from 192.168.2.0/24: