How do you write an ACL for cisco routers?

Access-lists are used for limiting the traffic say mainly for security purpose. Using access-list you can deny or permit the traffic from source address based on standard access list or deny or permit the traffic by classification of source or destination ip address, or by protocols . these are written in sequential order. So order of statement is very important . We should be clear of what to permit or deny .. sample config Standard access-list Router(config)#access-list 5 <permit> or <deny> <source ip address> Extended access-list Router(config)#access-list 111 <permit> or <deny> <protocol> <source ip address> <destination ip address> After creating access-list it will not be useful without applying on the interface where the traffic runs along the path. these should be applied in inbound direction or outbound direction of the interface as per our requirement. Most importantly when u want make changes , copy to text editor or notepad , do changes on them and remove the access-list config in the router and copy them fresh from the notepad.