ACL-Access List

Access List:

        It is used for filtering or blocking IP address.There are three types of access lists

1.       Standard Access List

                    It provides packet based filtering.Standard access list number range from 1-99.It is configured on destination router.

Configuration:

For blocking particular IP: Router(config)# access-list  <acl num> deny  host <ip address>

For blocking entire networks:  Router(config)# access-list  <acl num> deny  <source ip>

Router(config) access-list <acl num> permit any

Applying to interface: router(config) interface f0/0

Router(config-if)# ip access-group <acl num> in/out –it changes according to the scenario.

2.       Extended Access List:

              Number range from 100-199 and it provides protocol based filtering.we can configure ACL in source router or destination router but better on source router.By configuring extended ACL we can block communication from an IP to IP,network,ip to network,network to IP.

Configuration:

For blocking telnet and protocol

     Router(config)# access-list <acl num> deny <protocol> <source ip> <destination ip> eq <port num>

Ping blocking:

Router(config)# access-list 101 deny ICMP host<ip address> host<destination ip address>

Router(config)# access-list 101 permit ip any any

Applying to interface:

Router(config)# interface f0/0

Router(config-if)# ip access-group <acl num> in

3.       Extended Access list:

Router(config)# ip access-list extended “name”

Router(config-ext-nacl)# deny ip  host <source IP>host<destination IP>