Hi,
An ACL will blocked all the traffic that is not explicitly permitted.
An example of an ACL to allow only 5 PCs to talk to a server and then permit all PCs to surf the web:
ip access-list extended EXAMPLE
permit ip host 1.1.1.1 host 5.5.5.5
permit ip host 1.1.1.2 host 5.5.5.5
permit ip host 1.1.1.x host 5.5.5.5
deny ip 1.1.1.0 0.0.0.255 host 5.5.5.5
permit ip 1.1.1.0 0.0.0.255 any
The above example uses the PCs to be part of the 1.1.1.0/24 and the server 5.5.5.5
Federico.