Security MCQs with Explanations

A Cisco router has rules regarding the placement of access lists on a router interface. You can place one access list per direction for each layer 3 protocol configured on an interface.

To apply an access list, the proper command is ip access-group 101 in .

Standard IP access lists use the numbers 1-99 and 1300-1999 and filter based on source IP address only. Option C is incorrect because the mask must be in wildcard format.

The range of 192.168.160.0 to 192.168.191.0 is a block size of 32. The network address is 192.168.160.0 and the mask would be 255.255.224.0, which for an access list must be a wildcard format of 0.0.31.255. The 31 is used for a block size of 32. The wildcard is always one less than the block size.

The only command that shows which access lists have been applied to an interface is show ip interface Ethernet 0 . The command show access-lists displays all configured access lists, and show ip access-lists displays all configured IP access lists, but neither command indicates whether the displayed access lists have been applied to an interface.

When trying to find the best answer to an access-list question, always check the access-list number and then the protocol. When filtering to an upper-layer protocol, you must use an extended list, numbers 100-199 and 2000-2699. Also, when you filter to an upper-layer protocol, you must use either tcp or udp in the protocol field. If it says ip in the protocol field, you cannot filter to an upper-layer protocol. SMTP uses TCP.

First, you must know that a /20 is 255.255.240.0, which is a block size of 16 in the third octet. Counting by 16s, this makes our subnet 48 in the third octet, and the wildcard for the third octet would be 15 since the wildcard is always one less than the block size.

Only the show ip interface command will tell you which interfaces have access lists applied. show access-lists will not show you which interfaces have an access list applied.

Using a named access list just replaces the number used when applying the list to the router's interface. ip access-group Blocksales in is correct.

First, you must know that a /21 is 255.255.248.0, which is a block size of 8 in the third octet. Counting by eight, this makes our subnet 144 in the third octet, and the wildcard for the third octet would be 7 since the wildcard is always one less than the block size.

If you add an access list to an interface and you do not have at least one permit statement, then you will effectively shut down the interface because of the implicit deny any at the end of every list.

First, you must know that a /19 is 255.255.224.0, which is a block size of 32 in the third octet. Counting by 32, this makes our subnet 192 in the third octet, and the wildcard for the third octet would be 31 since the wildcard is always one less than the block size.

The extended access list ranges are 100-199 and 2000-2699, so the access-list number of 100 is valid. Telnet uses TCP, so the protocol TCP is valid. Now you just need to look for the source and destination address. Only the third option has the correct sequence of parameters. Answer B may work, but the question specifically states "only" to network 192.168.10.0, and the wildcard in answer B is too broad.

The first thing to check in a question like this is the access-list number. Right away, you can see that the second option is wrong because it is using a standard IP access-list number. The second thing to check is the protocol. If you are filtering by upper-layer protocol, then you must be using either UDP or TCP; this eliminates the fourth option. The third and last answers have the wrong syntax.

The wildcard 0.0.0.0 tells the router to match all four octets. This wildcard format alone can be replaced with the host command.

Extended IP access lists use numbers 100-199 and 2000-2699 and filter based on source and destination IP address, protocol number, and port number. The last option is correct because of the second line that specifies permit ip any any. (I used 0.0.0.0 255.255.255.255, which is the same as the any option.) The third option does not have this, so it would deny access but not allow everything else.

Telnet access to the router is restricted by using either a standard or extended IP access list inbound on the VTY lines of the router. The command access-class is used to apply the access list to the VTY lines.

To place an access list on an interface, use the ip access-group command in interface configuration mode.