CISCO NETWORKING ACADEMY Chabot College ELEC 99.08 Access Control Lists - Introduction.

CISCO NETWORKING ACADEMYCISCO NETWORKING ACADEMY

Chabot CollegeChabot College

ELEC 99.08ELEC 99.08Access Control Lists - IntroductionAccess Control Lists - Introduction


ACL TopicsACL Topics

• Function of ACLs

• ACL Types & Syntax

• Wildcard Bitmasks

• Placement of ACLs

• Commands


Typical FunctionsTypical Functions

• Security

• Firewalling


TypesTypes

• Standard

• Extended


Standard ACLsStandard ACLs

• Use rules based only the packet’s source address

• 1-99


Extended ACLsExtended ACLs

• Provide more precise (finer tuned) packet selection based on:– Source and destination addresses– Protocols– Port numbers

• 100-199


Steps to Configure ACLsSteps to Configure ACLs

1) Create ACL (global config mode)– The list may contain many rules, each on one line.– The list is identified by a number or name.

2) Apply to an interface (interface config mode)


How do ACLs work?How do ACLs work?

• Processing occurs line by line from top to bottom of the list.

• Each line tests a packet for a “match”.• If there is a match, a “permit” or “deny” rule is

applied.• When a “match” occurs, no further rules are

checked.• Invisible last line of an ACL is an implicit “deny

any.”


How do ACLs work?How do ACLs work?

• ACL example:

oak#sh ruoak#... oak#access-list 10 deny 192.168.1.0 0.0.0.255 oak#access-list 10 permit any oak#access-list 10 deny any (implicit)oak#...


How does a Standard ACL How does a Standard ACL work?work?• Permits or denies if source IP address

is matched:– Permit – packet is allowed – Deny – packet is dropped– Implicit Deny – If a packet’s address does

not match an earlier statement, an implicit deny any occurs at the end of every ACL and the packet is dropped.


Wildcard MasksWildcard Masks

• Are used to specify (by bits) the part of the ip address to be matched.

• Looks like a subnet mask but it its not!

• Example:172.16.0.0 0.0.255.255

The network address to be matched The wildcard bitmask



• Specify the part of the ip address to be matched.

• Use 0s to match,1s to ignore. (Reverse of subnet masks!)

• In the example below, only the 1st 2 octets will be examined for a match:172.16.0.0 0.0.255.255

Match this part of the address This is the wildcard bitmask



10101100 00010000 00000000 00000000

172 16 0 0Address

Wildcard Mask

11111111

Ignore

1111111100000000

2552550 0

Check for a match

00000000

172.16.0.0 0.0.255.255wildcard bitmaskaddress to match



• In this example, which octets will be examined for a match?172.16.5.0 0.0.0.255




• The first 3:172.16.5.0 0.0.0.255

Match this part of the address




• All 4 octets:172.16.5.2 0.0.0.0

Match the entire address (permit or deny this specific host)



• In Cisco 2, we will work only with wildcard bitmasks that are 0 or 255 for an entire octet.

• In Cisco 3, you’ll work with masks where the change from 0 to 1 does not fall on an octet boundary: – e.g. 0.0.15.255


Keyword: “any”Keyword: “any”

• Identical statements – access-list 22 permit 0.0.0.0 255.255.255.255– access-list 22 permit any


Keyword: “host”Keyword: “host”

• Identical statements– Access-list 23 permit 172.16.1.1 0.0.0.0– Access-list 23 permit host 172.16.1.1


Standard IP ACL commandStandard IP ACL command

access-list ACL-number {permit |deny} source-ip-address wildcard-mask

• ACL number: 1-99• Global Config mode


Standard ACL ExampleStandard ACL Example

• To permit all packets from the network number 172.16.0.0access-list 20 permit 172.16.0.0 0.0.255.255



• To permit traffic from the host 172.16.1.1 onlyaccess-list 20 permit 172.16.1.1 0.0.0.0

OR

access-list 20 permit host 172.16.1.1



• To permit traffic from any source address.access-list 20 permit 0.0.0.0 255.255.255.255

OR

access-list 20 permit any


How does an Extended ACL How does an Extended ACL work?work?• Permits or denies if all conditions match:

– Source Address– Destination Address– Protocol– Port No. or Protocol Options


Extended IP ACL commandExtended IP ACL command

access-list ACL-number {permit|deny} protocol source-ip-address source-wildcard-mask destination-ip-address destination-wildcard-mask eq port-number

• ACL number: 100-199• Global Config mode


Extended ACL ExampleExtended ACL Example

• To permit traffic from the network 192.168.1.0 to the host 192.168.3.10 only on telnet:

access-list 101 permit tcp 192.168.1.0 0.0.0.255 192.168.3.10 0.0.0.0 eq telnet

• More about extended ACLs later...


Major differencesMajor differences

• Standard ACL– Use only source address– Requires fewer CPU cycles.– Place as close to destination as possible.

(because they can only check source address)

• Extended ACL– Uses source, destination, protocol, port– Requires more CPU cycles.– Place as close to source as possible.

(This stops undesired traffic early.)


Command to apply IP ACLCommand to apply IP ACL

ip access-group ACL-number {in |out}• Interface Config mode• The group of rules in the list is applied to the

interface being configured.• Use “in” and “out” as if looking at the interface

from inside the router.


Do I place an ACL in?Do I place an ACL in?

• In – Coming into the router.– Requires less CPU processing because

every packet bypasses processing before it is routed.

– Filtering decision is made prior to the routing table.


Do I place an ACL out?Do I place an ACL out?

• Out – Going out of the router.– Routing decision has been made and the

packet is switched to the proper outbound interface before it is tested against the access list.

– ACLs are outbound unless otherwise specified.


ACL Configuration ExampleACL Configuration Example

oak(config)#access-list 10 permit 192.168.1.0 0.0.0.255oak(config)#access-list 10 permit 192.168.2.10 0.0.0.0oak(config)#int e0oak(config-if)#ip-access group 10 outoak(config-if)#^z

fre hay oak

192.168.3.0

E0S0

S1S0S1

What will this list do?

E0E0

192.168.2.0

192.168.1.0

192.168.1.10 192.168.2.10 192.168.3.10


ACL Configuration ExampleACL Configuration Example

oak(config)#access-list 10 permit anyoak(config)#access-list 10 deny 192.168.2.10 0.0.0.0oak(config)#int e0oak(config-if)#ip-access group 10 outoak(config-if)#^z

fre hay oak

192.168.3.0

E0S0

S1S0S1

What’s the problem here?

E0E0

192.168.2.0

192.168.1.0

192.168.1.10 192.168.2.10 192.168.3.10


Commands to show ACLsCommands to show ACLs

show access-lists• Privileged exec mode• Displays the ACLs on the router.

show ip interface• Privileged exec mode• Shows which ACLs are set on that interface.

CISCO NETWORKING ACADEMY Chabot College ELEC 99.08 Access Control Lists - Introduction.

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