Extended Access-List example on Cisco Router

In a previous lesson I covered the standard access-list, now it’s time to take a look at the extended access-list. This is the topology we’ll use:

standard access list example

Using the extended access-list we can create far more complex statements. Let’s say we have the following requirement:

  • Traffic from network 1.1.1.0 /24 is allowed to connect to the HTTP server on R2, but they are only allowed to connect to IP address 2.2.2.2.
  • All other traffic has to be denied.

Now we need to translate this to an extended access-list statement. Basically they look like this:

[source] + [ source port] to [destination] + [destination port]

Let’s walk through the configuration together:

R2(config)#access-list 100 ?
  deny     Specify packets to reject
  dynamic  Specify a DYNAMIC list of PERMITs or DENYs
  permit   Specify packets to forward
  remark   Access list entry comment

First of all we need to select a permit or deny. By the way you can also use a remark. You can use this to add a comment to your access-list statements. I’ll select the permit…

R2(config)#access-list 100 permit ?
  <0-255>  An IP protocol number
  ahp      Authentication Header Protocol
  eigrp    Cisco's EIGRP routing protocol
  esp      Encapsulation Security Payload
  gre      Cisco's GRE tunneling
  icmp     Internet Control Message Protocol
  igmp     Internet Gateway Message Protocol
  ip       Any Internet Protocol
  ipinip   IP in IP tunneling
  nos      KA9Q NOS compatible IP over IP tunneling
  ospf     OSPF routing protocol
  pcp      Payload Compression Protocol
  pim      Protocol Independent Multicast
  tcp      Transmission Control Protocol
  udp      User Datagram Protocol

Now we have a lot more options. Since I want something that permits HTTP traffic we’ll have to select TCP. Let’s continue:

R2(config)#access-list 100 permit tcp ?
  A.B.C.D  Source address
  any      Any source host
  host     A single source host

Now we have to select a source. I can either type in a network address with a wildcard or I can use the any or host keyword. These two keywords are “shortcuts”, let me explain:

  • If you type “0.0.0.0 255.255.255.255” you have all networks. Instead of typing this we can use the any keyword.
  • If you type something like “2.2.2.2 0.0.0.0” we are matching a single IP address. Instead of typing the “0.0.0.0” wildcard we can use the keyword host.

I want to select network 1.1.1.0 /24 as the source so this is what we will do:

R2(config)#access-list 100 permit tcp 1.1.1.0 0.0.0.255 ?
  A.B.C.D  Destination address
  any      Any destination host
  eq       Match only packets on a given port number
  gt       Match only packets with a greater port number
  host     A single destination host
  lt       Match only packets with a lower port number
  neq      Match only packets not on a given port number
  range    Match only packets in the range of port numbers

Besides selecting the source we can also select the source port number. Keep in mind that when I connect from R1 to R2’s HTTP server that my source port number will be random so I’m not going to specify a source port number here.

R2(config)#access-list 100 permit tcp 1.1.1.0 0.0.0.255 host 2.2.2.2 ?
  ack          Match on the ACK bit
  dscp         Match packets with given dscp value
  eq           Match only packets on a given port number
  established  Match established connections
  fin          Match on the FIN bit
  fragments    Check non-initial fragments
  gt           Match only packets with a greater port number
  log          Log matches against this entry
  log-input    Log matches against this entry, including input interface
  lt           Match only packets with a lower port number
  neq          Match only packets not on a given port number
  precedence   Match packets with given precedence value
  psh          Match on the PSH bit
  range        Match only packets in the range of port numbers
  rst          Match on the RST bit
  syn          Match on the SYN bit
  time-range   Specify a time-range
  tos          Match packets with given TOS value
  urg          Match on the URG bit
  <cr>

We will select the destination which is IP address 2.2.2.2. I could have typed “2.2.2.2 0.0.0.0” but it’s easier to use the host keyword. Besides the destination IP address we can select a destination port number with the eq keyword:

R2(config)#access-list 100 permit tcp 1.1.1.0 0.0.0.255 host 2.2.2.2 eq 80

This will be the end result. Before we apply it to the interface I will add one useful extra statement:

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Forum Replies

  1. Hi Rene,

    One quick question, why do you need to specify: Robocop(config)#access-list 100 deny ip any any log
    when at the end of every access list there is the invisible deny command.

    Any clarification would be greatly appreciated.

    Thanks

    P.S. Keep up the good work.

  2. Hi Edmundo,

    Adding the log keyword will show all denied packets in your console. This is useful for troubleshooting, debugging or labbing.

  3. So if I understand correctly we would still achieve the same results if without entering this command? And the only reason we would use this command is mostly for logging events, troubleshooting or labbing…correct?

    Thanks

  4. Yes that’s right. There is always a default deny any at the bottom. Give it a try on a Cisco IOS router…

  5. Hey… that’s really helpful…thnx so much…keep up the good work!

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