IP ACCESS LISTS

 

An access control list (ACL), with respect to a computer file system, is a list of permissions attached to an object. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects or you can say for each rule we have two conditions and that is Permit or Deny.

Types of Access Lists

There are two categories of access lists: numbered and named.

Numbered Access Lists:-

Numbered access lists are broken down into several ranges, each dedicated

to a specific protocol:

1–99 IP standard access list

100-199 IP extended access list

200-299 Protocol type-code access list

300-399 DECnet access list

400-499 XNS standard access list

500-599 XNS extended access list

600-699 Appletalk access list

700-799 48-bit MAC address access list

800-899 IPX standard access list

900-999 IPX extended access list

1000-1099 IPX SAP access list

1100-1199 Extended 48-bit MAC address access list

1200-1299 IPX summary address access list

1300-1999 IP standard access list (expanded range)

2000-2699 IP extended access list

Named Access Lists:-

Named access lists provide a bit more flexibility. Descriptive names can be

used to identify your access-lists. Additionally, individual lines can be

removed from a named access-list. However, like numbered lists, all new

entries are still added to the bottom of the access list.

There are two common types of named access lists:

• IP standard named access lists

• IP extended named access lists

Standard IP Access List

access-list [1-99] [permit | deny] [source address] [wildcard mask] [log]

Standard IP access-lists are based upon the source host or network IP address, and should be placed closest to the destination network.

Router(config)# access-list 10 deny 172.18.0.0 0.0.255.255 ( Just for an Example)

Router(config)# access-list 10 permit any

To apply Access Lists we have to configure the Access-Group on the Interface. Like wise we are taking the interface serial 0 as a refrence.

Router(config)# int s0

Router(config-if)# ip access-group 10 in

To view all IP access lists configured on the router:

Router# show ip access-list

To view what interface an access-list is configured on:

Router# show ip interface

Router# show running-config

Extended IP Access List

access-list [100-199] [permit | deny] [protocol] [source address] [wildcard

mask] [destination address] [wildcard mask] [operator [port]] [log]

Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80

Router(config)# access-list 101 deny ip 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255

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

*The above ip address is just taken for the example and dont have real environment existance.

The first line allows the 172.18.x.x network access only to port 80 on the web server. The second line blocks 172.18.x.x from accessing anything else on the 172.16.x.x network. The third line allows 172.18.x.x access to anything else.

To apply this access list, we would configure the following

Router(config)# int e0

Router(config-if)# ip access-group 101 in

Extended IP Access List Port Operators

In the preceding example, we identified TCP port 80 on a specific host use the following syntax:

Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80

We accomplished this using an operator of eq, which is short for equals. Thus, we are identifying host 172.16.10.10 with a port that equals 80. We can use several other operators for port numbers:

• eq Matches a specific port
• gt Matches all ports greater than the port specified
• lt Matches all ports less than the port specified
• neq Matches all ports except for the port specified
• range Match a specific inclusive range of ports

ICMP Access List
The specific ICMP port that a “ping” uses is echo. To block specific ICMP parameters, use an extended IP access list. On Router B, we would configure:

Router(config)# access-list 102 deny icmp 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255 echo
Router(config)# access-list 102 permit icmp 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255
Router(config)# access-list 102 permit ip any any

The first line blocks only ICMP echo requests (pings). The second line allows all other ICMP traffic. The third line allows all other IP traffic.

To apply the access lists on other router, you need to configure the follwing as:-

Router(config)# int e0
Router(config-if)# ip access-group 102 in

Named Access-Lists will be cover in the Next Session.

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