Access Control

CCNA4

Perrine /Brierley Page 104/20/23

Chapter 05Access Control

172.16.3.0 172.16.4.0

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e0 e1

s0

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Chapter 05Access Control

172.16.2.0 172.16.4.0

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s0172.16.4.3

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Computer

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172.16.2.2

172.16.3.1

s1

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172.16.4.2

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Chapter 05

A HIGH LEVEL VIEW of ACLs

High Level View

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Chapter 05

Access Control Lists

Standard

Extended

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Chapter 05Standard Access Control Lists

ACLs are instructions that are applied to a router interfaces.

The ACLs describe the kind of packets that are to be permitted or denied.

Permitted or Denial testing can be based on:

• source address

• destination address

• port number

ACLs are configured on the router Interfaces to control access to a network.

ACLs must be defined separately for each protocol; Unique for IP, Unique for IPX; Unique for AppleTalk, etc.

Some times ACLs are called packet filters.

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Chapter 05Standard Access Control Lists

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Chapter 05Extended ACLs

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Chapter 05Named ACLs

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Chapter 05

• Standard ACLs should be placed close to the destination.• Extended ACLs should be placed close to the source.

Placing ACLs

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Chapter 05

A firewall is an architectural structure that exists between the user and the outside world to protect the internal network from intruders.

Firewalls

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Chapter 05Restricting Virtual Terminal Access

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Chapter 05Summary

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Chapter 05

Reasons to create ACLs:

• limit network traffic - hence increase network performance

• provide traffic flow - limit traffic through the network

• provide for security

• ACLs establish

•which traffic is blocked

•which traffic is not blocked

ACL - Access Control Lists

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Chapter 05

A MORE DETAILED VIEW of ACLs

DETAIL

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Chapter 05

Cisco IOS checks each packet for:

1. destination address

2. source address

3. protocol

4. port number

Each ACL statement is checked in a sequential order (first to last) and when there is a match, no more statements are checked.

If the results are no matches, then the packet (by default) is discarded.

Adding addition ACL statements to the end of an existing list is just a matter of adding the new statement. BUT, if

deleting an existing ACL statement causes the entire access list to be deleted.


access-list 1 deny 192.169.1.0 0.0.255.255access-list 1 deny 192.168.1.9 0.0.0.0access-list 1 deny 172.16.4.0 0.0.0.255access-list 1 permit any

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Chapter 05

When a packet enters a router’s interface:

“IN” (inbound) checking is required. The packet:

1) Is checked against the ACL list (if one exists)

2) Here It can be interrogated to permit or deny.

3) If denied the packet is dropped else,

4) It is matched against the routing table and passed to an “OUT” (outbound interface)


“OUT” (outbound) checking is required.1) Here It can be interrogated to permit or deny.

2) If denied the packet is dropped3) If permitted then packet is allowed “OUT” (outbound).

The Outbound interface’s ACL is a different list from the inbound)

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Chapter 05

ACL statements operate in a logical, sequential order. When a match is made the rest of the statements are not checked.

If none the ACL statements match, then there is an implicit deny any rule.

access-list 10 { permit | deny } { test conditions }

access-list 1 deny 192.169.1.0 0.0.255.255access-list 1 deny 192.168.1.9 0.0.0.0access-list 1 deny 172.16.4.0 0.0.0.255

access-list 1 permit any




access-list 10 deny anyaccess-list 10 deny any


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Chapter 05

Two types of IP ACL :

• standard - has access list value of 1- 99

• extended - has access list value of 100 - 199

Must be configured in global configuration mode. Router (config) #

Steps in creating ACLs:

1) create the ACL (in config mode)

2) apply the ACL to an interface

ACLs are used to filter:

• inbound traffic, or

• outbound traffic


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Chapter 05Where to place ACLs

Standard ACLs are place as close as possible to the destination.

Extended ACLs are place as close as possible to the source.

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Chapter 05

Configuring an Access List:

Router(config)# access-list <ACL number> { permit | deny } { test conditions }

Router(config)# access-list 1 permit { test conditions }

Router(config)# access-list 50 deny { test conditions }

To delete all ACL statements of an access-listRouter(config)# no access-list <ACL number>

Applying the Access List:

At an interface:

Router(config)# int E0

Router(config-if)# { protocol } access-group <ACL number> [IN | OUT]

Out is the default if not mentioned

Router(config-if)# ip access-group 1

Router(config-if)# ip access-group 50

To delete an ACL group statement (this will not delete the associated list):

Router(config)# no access-group <ACL number>

Where to place ACLs

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Chapter 05

A wildcard is matched with an IP address or protocol address.

It is a 32 bit mask divided into 4 octet, each containing 8 bits.

A 0 in the wildcard means to check the bit in the IP you are testing.

A 1 in the wildcard means ignore the bit in the IP you are testing.

NOTE!!!

Do NOT think subnet mask – that is a totally different meaning not related to the WILDCARD

Wildcard

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Chapter 05Abbreviations

To permit or deny any address:

0.0.0.0 255.255.255.255

Address Wildcard

Use the abbreviation any

Router(config)# access-list 1 permit 0.0.0.0 255.255.255.255

Router(config)# access-list 1 permit any

Router(config)# access-list 1 deny 0.0.0.0 255.255.255.255

Router(config)# access-list 1 deny any

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Chapter 05

To match all the bits of IP address use host:

EX: 172. 30.16. 29 0. 0. 0. 0

Router(config)# access-list 1 permit 172.30.16.29 0.0.0.0

Router(config)# access-list 1 permit host 172.30.16.29

Abbreviations

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Chapter 05Standard ACLs

Criteria:

• block all traffic from a network

• allow all traffic from a network

• deny entire protocol suits

Standard ACLs only check the source address.

Router(config)# access-list <ACL number> { deny | permit } source [ source wildcard] [log]

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Chapter 05

What does this statement accomplish?

Access-list 33 permit 172.16.0.0 0.0.255.255 log

Permits all traffic from 172.16.0.0 and sends messages to the console every time the access list is executed.

Standard ACLs

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Chapter 05


Access-list 44 deny 172.16.13.7 0.0.0.0 log

Denies traffic from host 172.16.13.7 and sends message to the console every time the access list is hit.

Standard ACLs

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Chapter 05


Access-list 55 deny 172.16.64.3 0.0.0.255

Denies all traffic from network 172.16.64.0

Standard ACLs

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Chapter 05

The log command:

Prints messages to the console which includes the ACL number, whether the packet was permitted or denied, the source address, and the number of packets.

The message is generated for the first packet that matches, and then at five-minute intervals, including the number of packets permitted or denied in the prior five-minute interval.

Log is used for debugging only not to be left active on live networks.

Standard ACLs

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Chapter 05

Example of applying the access-list:

Router(config-if)# ip access-group 33 in

Router(config-if)# ip access-group 44 out

Standard ACLs

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Chapter 05

To monitor IP access list: (in EXEC mode)

show access-list

Displays all access lists & their parameters configured on the router. (Does not show you which interface the list is set on.)

show access-list <ACL number>

Shows only the parameters for the access list <ACL number>. (Does not show you the interface the list is set on.)

show ip access-list

Shows only the IP access lists configured on the router

show ip interface

Shows which interfaces have access lists set (containing an access-group).

show running-config

Shows the routers entire configuration

Standard ACLs

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Chapter 05

R(config)# Interface e0R(config-if)# ip access-group 1 out


R(config)# access-list 1 permit 172.16.0.0 0.0.255.255

What does it do?

Standard ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

s0

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Chapter 05

R(config)# Interface e0

R(config-if)# ip access-group 1 out



R(config)# access-list 1 permit 172.16.0.0 0.0.255.255

Allows only traffic from source network 172.16.0.0 to be forwarded & and non-172.16.0.0 traffic is blocked.

Standard ACLs

One Access list used by multiple Interfaces

172.16.3.0 172.16.4.0

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Chapter 05



R(config)# access-list 1 deny 172.16.4.13 0.0.0.0

R(config)# access-list 1 permit any

What does this do?

Standard ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

s0

172.16.4.13

server

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Chapter 05


R(config)# access-list 1 deny 172.16.4.13 0.0.0.0R(config)# access-list 1 permit any

Denies traffic from a specific device, 172.16.4.13 & allows all other traffic thru e0 to network 172.16.3.0.

Standard ACLs

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Chapter 05

Interface e0

ip access-group 1 out

access-list 1 deny 172.16.4.0 0.0.0.255

access-list 1 permit any

What does this do?

Standard ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

s0

172.16.4.13

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Chapter 05

Interface e0ip access-group 1 out

access-list 1 deny 172.16.4.0 0.0.0.255access-list 1 permit any

Denies traffic from the subnet, 172.16.4.0 & allows all other traffic thru e0 to network 172.16.3.0.

Standard ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

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Chapter 05

Criteria:• checks both the packet’s source & destination addresses

• check for specific protocol

• check for specific port numbers

• permit or denied applications – pings, telnets, FTP, etc.

•ACL values range between 100 – 199 (for IP)

Extended ACLs

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Chapter 05

Port Numbers (decimal) IP Protocol

20 FTP data [TCP]

21 FTP program (control) [TCP]

23 Telnet [TCP]

25 Simple Mail Transport Protocol

[TCP] (SMTP)

53 DNS [TCP, UDP]

69 TFTP [UDP]

80 HTTP [TCP]

Extended ACLs

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Chapter 05

Router(config)# access-list <ACL number> { permit | deny } protocol source source-mask

destination destination-mask operator operand {established}

ACL number100 – 199

permit | denyPacket is allowed or blocked

protocol IP, TCP, UDP, ICMP, GRE or IGRP

source -- Source address source-wildcard mask

destination -- Destination address destination-wildcard mask

Extended ACLs

Continued

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Chapter 05

Router(config)# access-list <ACL number> { permit | deny } protocol source source-mask destination destination-mask operator

operand {established}

operatorlt, gt, eq, neq

OperandPort number

established Allows TCP traffic to pass if the packet uses an established connection ( for example, has ACK bits set ).

access-list 101 permit tcp 172.16.4.0 0.0.0.255 any eq 25

Extended ACLs

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Chapter 05

Configuring an extended ACL to an interface:

Router(config-if)# ip access-group <ACL number> { in | out }

Router(config)# int E0


Extended ACLs

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Chapter 05

Interface e0

ip access-group 101

access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 21

access-list 101 permit ip any any

What does this do?

Extended ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

s0

172.16.4.13

server

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Chapter 05

Interface e0ip access-group 101

access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 21access-list 101 permit ip any any

Blocks FTP traffic from all hosts on 172.16.4.0 to any device on 172.16.3.0 & allows all other traffic.

Extended ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

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Chapter 05

Interface e0

ip access-group 101

access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 23

access-list 101 permit ip any any

What does this do?

Extended ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

s0

172.16.4.13

server

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Chapter 05

Interface e0ip access-group 101

access-list 101 deny tcp 172.16.4.0 0.0.0.255 172.16.3.0 0.0.0.255 eq 23access-list 101 permit ip any any

Denies only telnet traffic from 172.16.4.0 to 172.16.3.0 network, and permits all other traffic thru e0 to any address.

Extended ACLs

172.16.3.0 172.16.4.0

Non-172.16.0.0

e0 e1

s0

172.16.4.13

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Chapter 05Extended/Standard ACL numbers for IP

NOTE:

Standard ACL numbers: 1-99; 1300-1999

Extended ACL numbers: 100-199; 2000-2699

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Chapter 05Standard/Extended ACL

You can not add ACL statements into the body of the access-list (ONLY at the end of the list).

Otherwise the access list must be deleted first, and then rewritten.

Therefore it is prudent to write your access-list in text format using “notepad”, and then transfer it to your router.

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Chapter 05Configuring Named ACLs

NOTE:

• A NAMED ACL is an alphanumeric string instead of the ACL number (1 - 199 )

• NAMED ACLs are not compatible with Cisco IOS release prior to Release 11.2

• Named ACLs can be used for either standard & extended

•You cannot configure the same name for multiple ACLs.

•use Name ACL when you want to intuitively identify ACLs

•use Name ACL when you have more than 99 standard & 100 extended ACLs have been configured on a router for a given protocol

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Chapter 05Configuring Named ACLs

Router(config)# ip access-list { standard | extended } name { deny | permit } { commands }

ip access-list standard internetfilter

deny 172.10.15.0 0.0.0.255

permit 128.88.0.0 0.0.255.255

permit 36.0.0.0 0.0.255.255

ip access-list extended marketing-group

permit tcp any 171.69.0.0 0.255.255.255 eq telnet

deny udp any 171.69.0.0 0.255.255.255 lt 1024

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Chapter 05Named ACL

A named ACL will allow the deletion of statements, but will only allow for the statements to be inserted a the end of the list.

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Chapter 05More Details

access-list 1 permit 172.16.0.0 0.0.255.255

10101100.00010000.00000000.00000000

00000000.00000000.xxxxxxxx. xxxxxxxx

10101100.00010000.xxxxxxxx. xxxxxxxx Matched value

Incoming packet with address of 172.18.4.2. Will it be permitted?

Source : 10101100.00010010.00000100.00000010

Wildcard mask: 00000000.00000000.xxxxxxxx. xxxxxxxx

10101100.00010010.xxxxxxxx. xxxxxxxx Result

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Chapter 05


10101100.00010000.00000000.00000000


10101100.00010000.xxxxxxxx. xxxxxxxx Match value


Source : 10101100.00010010.00000100.00000010



Does the result equal the match value?

No! Hence the incoming packet will not be permitted.

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Chapter 05


10101100.00010000.00000000.00000000


10101100.00010000.xxxxxxxx. xxxxxxxx Match value


Source : 10101100.00010000.00000100.00000010



Does the result equal the match value?

Yes! Hence the incoming packet will be permitted.

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Chapter 05


10101100.00010000.00000000.00000000

00000000.00000000.xxxxxxxx. xxxxxxx0

10101100.00010000.xxxxxxxx. xxxxxxx0 Matched value


Source : 10101100.00010000.00000100.00000001

Wildcard mask: 00000000.00000000. xxxxxxxx. xxxxxxx0

10101100.00010000.xxxxxxxx. xxxxxxx1 Result


Source : 10101100.00010000.00000100.00000100

Wildcard mask: 00000000.00000000. xxxxxxxx. xxxxxxx0

10101100.00010000. xxxxxxxx. xxxxxxx0 Result

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Chapter 05


10101100.00010000.00000000.00000000

00000000.00000000.xxxxxxxx. xxxxxxx0

10101100.00010000.xxxxxxxx. xxxxxxx0 Matched value


Source : 10101100.00010000.00000100.00000101

Wildcard mask: 00000000.00000000.xxxxxxxx. xxxxxxx0

10101100.00010000.xxxxxxxx. xxxxxxx1 Result

So the access list perform what operation? Permits 172.16.4.4, and denies 172.16.4.1 and 172.16.4.5

Permits all even addresses from the network 172.16.0.0

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Chapter 05Permit/Deny BLOCKS of addresses

One can permit or deny a block of addresses. However, the blocks must be a power of 2! (Example, 2, 4, 8, 16, 32, 64, 128, etc.)

When you need to specify a range of addresses, you choose the closet block size for your needs.

You want to block access to part of network that is in the range from 198.16.99.0 through 198.16.99.7. This is a block size of 8. Hence:

198.16.99.0 0.0.0.7

Also in this case for a block of 8, the beginning address must either start at 0, 8, 16, etc.

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Chapter 05

One has a subnet whose addresses range from 200.17.2.128 to 200.17.2.191. One wants to divide this network so the top half are permitted and the bottom half is denied to any other network. What is the access lists?

The block range is:

64

What is the access-list for the bottom?

access-list 101 deny ip 200.17.2.128 0.0.0.31 any

What is the access-list for the top?

access-list 101 permit ip 200.17.2.160 0.0.0.31 any

Permit/Deny BLOCKS of addresses

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Chapter 05

What does this do?

access-list 10 deny 200.16.88.64 0.0.0.63

Denies a block of 64 address starting at 200.16.88.64

Permit/Deny BLOCKS of addresses

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Chapter 05Virtual Terminal ACL

You can control access via the VTY ports controlling telnet sessions coming into the router.

You write the ACL as usual, but use access-class to apply it.

As an example:

Router(config t)# access-list 1 permit 172.16.1.0 0.0.0.255

Router(config t)# line vty 0 4

Router(config-line)# login

Router(config-line)# password cisco

Router(config-line)# access-class 1 in

Note: only numbered access lists can be applied to VTY virtual lines!

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Chapter 05Established option

The ‘establish’ option in an access-list used only with TCP datagrams. There are cases when you want to stop host B from initiating a connection with a host A while permitting A to initiate connections with B.

Macintosh SE¾

Macintosh SE¾

establish

response

establish

A B

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Chapter 05

Allow host 172.16.3.13 with Internet connection, but don’t allow the internet to initialize any sessions.

172.16.3.0

172.16.4.0

e0 e1

172.16.3.13

INTERNET

As a practical example:

Established option

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Chapter 05

Router(config)# int e1


Router(config)# access-list 101 permit tcp any 172.16.3.0 0.0.255.255 established

Established option

172.16.3.0

172.16.4.0

e0 e1

172.16.3.13

INTERNET

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Chapter 05



Router(config)# access-list 101 permit tcp any host 172.16.3.13 eq www established

Established option

172.16.3.0

172.16.4.0

e0 e1

172.16.3.13

INTERNET

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Chapter 05



Router(config)# access-list 101 permit tcp any 172.16.3.0 0.0.0.255 eq www established

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

Router(config)# access-list 101 permit udp any any eq 53

Note: established argument is limited to tcp which means UDP, ICMP and all other IP protocols will not match, and will be denied, unless specifically allowed. Hence

Established option

172.16.3.0

172.16.4.0

e0 e1

172.16.3.13

INTERNET

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Chapter 05Where to Place ACLs?

ACL Rules:

Standard ACL

Place the ACL as near the destination as possible.

Extended ACL

Put the ACL as close as possible to the source

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Chapter 05

Access ListsStandard

Extended

End of Session

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