Content delivery network

Here you will find answers to access control list questions

Question 1:

A network administrator wants to add a line to an access list that will block only Telnet access by the hosts on subnet 192.168.1.128/28 to the server at 192.168.1.5. What command should be issued to accomplish this task?

A – access-list 101 deny tcp 192.168.1.128 0.0.0.15 192.168.1.5 0.0.0.0 eq 23
access-list 101 permit ip any any
B – access-list 101 deny tcp 192.168.1.128 0.0.0.240 192.168.1.5 0.0.0.0 eq 23
access-list 101 permit ip any any
C – access-list 1 deny tcp 192.168.1.128 0.0.0.255 192.168.1.5 0.0.0.0 eq 21
access-list 1 permit ip any any
D – access-list 1 deny tcp 192.168.1.128 0.0.0.15 host 192.168.1.5 eq 23
access-list 1 permit ip any any

Answer: A

Explanation:

First the question asks to block only Telnet access so the port we have to use is 23 -> C is not correct.

Next we need to block traffic from hosts on the subnet 192.168.1.128/28, which is 192.168.1.128 0.0.0.15 if we convert to wildcard mask (just invert all bits of the subnet mask,from 0 to 1 and from 1 to 0 we will get the equivalent wildcard mask of that subnet mask) -> so B is incorrect

In this case, we have to use extended access list because we need to specify which type of traffic (TCP) and which port (23) we want to block -> so D is incorrect because it uses standard access list.

Question 2:

As a network administrator, you have been instructed to prevent all traffic originating on the LAN from entering the R2 router. Which the following command would implement the access list on the interface of the R2 router?

A – access-list 101 in
B – access-list 101 out
C – ip access-group 101 in
D – ip access-group 101 out

Answer: C

Question 3:

The following access list below was applied outbound on the E0 interface connected to the 192.169.1.8/29 LAN:

access-list 135 deny tcp 192.169.1.8 0.0.0.7 eq 20 any
access-list 135 deny tcp 192.169.1.8 0.0.0.7 eq 21 any

How will the above access lists affect traffic?

A – FTP traffic from 192.169.1.22 will be denied
B – No traffic, except for FTP traffic will be allowed to exit E0
C – FTP traffic from 192.169.1.9 to any host will be denied
D – All traffic exiting E0 will be denied
E – All FTP traffic to network 192.169.1.9/29 will be denied

Answer: D

Explanation:

There is always an implicit “deny all” command at the end of every access list, so if an access list doesn’t have any “permit” command, it will block all the traffic. If we use the command “access-list 135 permit ip any any” at the end of this access list then the answer should be C – FTP traffic from 192.169.1.9 to any host will be denied.

Question 4:

The access control list shown in the graphic has been applied to the Ethernet interface of router R1 using the ip access-group 101 in command. Which of the following Telnet sessions will be blocked by this ACL? (Choose two)

A – from host PC1 to host 5.1.1.10
B – from host PC1 to host 5.1.3.10
C – from host PC2 to host 5.1.2.10
D – from host PC2 to host 5.1.3.8

Answer: B D

Explanation:

Below is the simple syntax of an extended access list:

access-list access-list-number {deny | permit} {ip|tcp|udp|icmp} source [source-mask] dest [dest-mask] [eq dest-port]

Notice that this access list is applied to the Ethernet interface of R1 in the “in direction” so in this case, it will filter all the packets originated from E1 network (host PC1 and PC2) with these parameters:

Source network: 5.1.1.8 0.0.0.3 which means 5.1.1.8/252 (just invert all the wildcard bits to get the equivalent subnet mask) -> Packets from 5.1.1.8 to 5.1.1.11 will be filtered.

Destination network: 5.1.3.0 0.0.0.255 which means 5.1.3.0/24-> Packets to 5.1.3.0/24 will be filtered

Therefore packets originated from 5.1.1.8 to 5.1.1.11 and have the destination to the host 5.1.3.x (via Telnet) will be denied.

Question 5:

The following configuration line was added to router R1

Access-list 101 permit ip 10.25.30.0 0.0.0.255 any

What is the effect of this access list configuration?
A – permit all packets matching the first three octets of the source address to all destinations
B – permit all packet matching the last octet of the destination address and accept all source addresses
C – permit all packet matching the host bits in the source address to all destinations
D – permit all packet from the third subnet of the network address to all destinations

Answer: A