There are two types of access control lists (ACLs): standard ACLs and extended ACLs. Standard ACLs filter based only on the source IP address and are less processor intensive, while extended ACLs allow filtering on source/destination IP addresses, protocols, and ports to provide more granular control but require more processing power. Both can be used to restrict access to sensitive servers and resources like allowing only management network users to access an important documents server.

1. Types of ACLs

2.  There are two types of access lists:
1. standard access lists -
With standard access lists, you can filter only on the source
IP address of a packet.
These types of access list are not as powerful as extended
access lists, but they are less processor intensive for the
router.

3. The following example describes the way in which standard
access lists can be used.

4.  Let's say that server S1 holds some important documents
that need to be available only to company's management.
 We could configure an access list on R1 to enable access to
S1 only to users from the management network.
 All other traffic going to S1 will be blocked.
 This way, we can ensure that only authorized user can
access sensitive files on S1.

5. 2. extended access lists -
With extended access lists, you can be more precise in your
filtering.
You can evaluate source and destination IP addresses, type
of layer 3 protocol, source and destination port... Extended
access lists are harder to configure and require more
processor time than the standard access lists, but they allow
a much more granular level of control.

6. To demonstrate the concept, we will use the following
example.

7.  We have used the standard access list to prevent all users to
access server S1.
 But, with that configuration, we have also disable access to
S2! To be more specific, we can use extended access lists.
Let's say that we need to prevent users from accessing
server S1.
 We could place an extended access list on R1 to prevent
users from accessing S1.
 That why, no other traffic is forbidden, and users can still
access the other server, S2: