1. Access Control Lists CCNA 2 v3 – Module 11
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CCNA2v3 Module 11 DC

2. What are ACLs? ACLs provide basic traffic filtering capabilities.
ACLs enable management of traffic and secure access to and from a network.
ACLs can be created for various network protocols; IP, IPX, or AppleTalk.
The router examines each packet and either forwards or drops it.
This router has one _________ interface and one ________ interface.
Each interface can have one ACL __________ and one ACL __________
This router is configured for three routed protocols:
Example:
Ethernet E0
Serial S0
outbound
inbound
IP, AppleTalk, IPX.
Maximum no. of ACLs that can be configured:
(No. of interfaces) x 2 x (no. of routed protocols) = 12
Forwarding decision can be based on a packets:
Source address
Destination address
Protocol
Port number
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3. Why use ACLS? Limit network traffic to increase network _____________.
Provide traffic flow control by restricting the delivery of ________________.
__________ - allow one host access but prevent another.
Control which types of traffic are __________ or ________ by the router.
Ability to control which _______ a client can access.
Restrict user access to only certain _____________, (eg. Web pages).
If ACLs are not configured, __________ passing through the router will be allowed onto _________ of the network.
performance
routing updates
Security
forwarded
blocked
areas
types of files
all packets
all parts
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4. How ACLs Work
ACL statements operate in sequential, logical order, from top to bottom.
If a condition is matched, the packet is permitted or denied and the rest of the ACL isn’t checked.
An implicit "deny any" statement is at end of list by default.
This last line "deny any" is not visible but it will not allow any unmatched packets to be permitted.
ACL on interface? No
Route packet to
outbound interface
Yes
Any matches?
Yes
Frame arrives
at inbound
interface
L2 address match?
Yes
ACL on interface? No
Default
Deny
Yes
Any matches? No
Permit?
Yes No
Default
Deny No
Permit?
Yes
Yes
Forward packet No

5. Creating ACLs There are many different types of ACLs.
Each ACL is uniquely identified by assigning a number (or a name) to it.
This number identifies the type of access list created and must fall within the specific range of numbers:   
Rio(config)#
access-list ?
<1-99> IP standard access list
< > IP extended access list
< > Protocol type-code access list
< > DECnet access list
< > Appletalk access list
< > bit MAC address access list
< > IPX standard access list
< > IPX extended access list
< > IPX SAP access list
< > Extended 48-bit MAC address access list
< > IPX summary address access list
< > IP standard access list (expanded range)
< > IP extended access list (expanded range)
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6. Applying ACLs
Creating the access list is the first half of using them on a router.
The second half of the process is assigning them to an interface.
Rio(config)#
interface fastethernet 0/0
Rio(config-if)#
ip access-group ?
<1-199> IP access list (standard or extended)
< > IP expanded access list (standard or extended)
WORD Access-list name
Rio(config-if)#
ip access-group 10 ?
in inbound packets
out outbound packets
From inside the router
A numbered ACL cannot be edited on the router.
To edit an ACL:
Copy it to a text file.
Remove from router configuration with ‘no’ form of ACL statement.
Make necessary changes to text file.
Paste back to global configuration mode.
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7. Wildcard Masks
Wildcard masks are 32 bits long and paired with an IP address.
Wildcard masks are used with ACLs to filter groups of IP addresses.
Example 1:
Rio(config)#
access-list 10 permit
By applying the WM to the IP address, the router will ignore the values of the last two octets.
This statement will permit traffic with source IP: _______________________
[anything].[anything]
Example 2:
Rio(config)#
access-list 10 permit
This WM discounts the value of the last octet, and 4 bits from the 3rd octet (moving right to left).
If M = Match D = Don’t Care, WM =
MMMMMMMM•MMMMMMMM•MMMMDDDD•DDDDDDDD
This statement will permit traffic from range ___________ to _____________
Two keywords used in ACLs:
any
host
- means an IP address of _______ and WM ______________
- matches an address exactly, or WM __________
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8. More general statement denying other traffic from 10.0.0.0/24 network
Standard ACLs
Standard IP ACLs check only the ______________ of packets to be routed.
source address
Rio(config)#
access-list 50
deny
Rio(config)#
access-list 50 permit
No WM specified,
mask =
Number between 1 and 99,
or 1300 to1999 (recent IOS)
Deny or Permit
Wildcard Mask
Extended ACLs
Extended ACLs check the ________ and ___________ packet addresses as well as being able to check for __________ and _____________.
source
destination
protocols
port numbers
Protocol
Source
Destination
Port 1.
access-list 101 permit ip host any
access-list 101 deny ip any
access-list 101 deny tcp host eq 23
access-list 101 permit tcp any eq telnet 2. 3. 4.
Specifically permit all IP traffic from this host (only) to any other network or host
More general statement denying other traffic from /24 network
Specifically denies host (only) telnet access to /24 network
More general statement permitting telnet from all other hosts on /24 network

9. Named ACLs
IP named ACLs were introduced in Cisco IOS Software Release 11.2.
Advantages are:
Intuitively identify ACLs using names (not just numbers).
Extend possibilities beyond 798 simple and 799 extended ACLs
Modification of a NACL without deleting and reconfiguring it.
NACLs allow individual statements to be deleted without losing whole list.
It is still only possible to add statements to the end of a list.
Standard or Extended
Administrator assigned name
Rio(config)#
ip access-list extended Server-Access
Rio(config-ext-nacl)#
permit tcp any host eq smtp
Rio(config-ext-nacl)#
permit udp any host eq 53
Rio(config-ext-nacl)#
[Control + Z]
Rio(config)#
interface f 0/0
Rio(config-if)#
ip access-group Server-Access out
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10. Placing ACLs Verifying ACLs Command Description
ACLs placed in proper location filter traffic and increase network efficiency.
The general rule is to put extended ACLs close to the source of denied traffic.
Standard ACLs do not specify destination addresses, so they should be placed as close to the destination as possible.
Administrators can only place ACLs on devices under their control.
Verifying ACLs
These show commands verify the content and placement of ACLs:
Command
Description
Interface information includes number of inbound or outbound ACL.
Lists contents of all ACLs on the router
Interface placement and ACL contents in current configuration
show ip interface
show access-list
show running-config
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11. ACL Rules One access list per ________, per _________, per _________.
Standard access lists should be applied closest to the ___________.
Extended access lists should be applied closest to the ________.
Use the inbound or outbound interface reference as if looking at the port from ________________.
The ______________ at the end of all access lists will not appear in the configuration listing.
Access list entries should filter in the order from specific to ________.
The permit or deny is examined ONLY if the _____________.
New lines are always added to the _____ of the access list.
___________________________ will remove the whole list.
It is not possible to selectively add and remove lines with _________ ACLs.
An IP ACL will send ____________________ to sender of rejected packet.
In some situations, removing an access list may result in a default ____________ being applied to the interface.
Outbound filters do not affect traffic originating at _____________.
protocol
interface
direction
destination
source
inside the router
implicit deny
general
match is true
end
no access-list [number]
numbered
ICMP host unreachable
‘deny any’
the local router
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12. Firewalls
A network firewall can be one or several machines working together to prevent unwanted access.
External
Internal
Web
Server
DMZ
FTP
Server
Firewalls control access to services both into and from the internal network.
ACLs are used in firewalls between the internal and external network.
A De-Militarised Zone contains network services available to Internet traffic.
The firewall router provides isolation for the internal network and the DMZ.
Border routers (at the edge of a network) use ACLs to provide security benefits.
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13. Restricting VTY Access
A router has both physical ports (Fa0/0, S0/0) and ________ ports.
These virtual ports are called __________.
There are five such vty lines, numbered __ to __ (0 to 15 on later IOS).
VTY access can be restricted on routers by using ____________.
Access to vty is accomplished using _________.
Identical restrictions should be placed on all vty lines as it is not possible to control _____________________________.
The process of creating a vty access list is the same as for ____________.
Applying the ACL to a terminal line (vty, aux or con) requires the command ____________ instead of _______________.    
Only ___________ ACLs can be applied to vty lines.
virtual
vty lines 4
access lists
Telnet
which line a user will connect on
an interface
access-class
access-group
numbered
Rio(config)#
line vty 0 4
Rio(config-line)#
login
Rio(config-line)#
password Cisco
Rio(config-line)#
access-class 2 in
Rio(config-line)#
end
Rio#
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