If the router RT1 has a packet with a destination address 192.168.1.255, which statement below describes the operation of the network?
A ? RT1 forwards the frame containing this packet, SW1 will add 192.168.1.255 to its MAC table
B ? RT1 will drop this packet because it is not a valid IP address
C ? RT1 will forward the packet out all interfaces
D ? RT1 will encapsulate the packet in a frame with a destination MAC address of FF-FF-FF-FF-FF-FF
E ? As RT1 forwards the frame containing this packet, SW1 will forward it ti the device assigned the IP address of 192.168.1.255
h2pass Answer: B
well, 192.168.1.255 might be not a valid HOST ip address, but it is still a valid broadcast IP address for the 192.168.1.0/24 network.
If directed broadcasts were enabled with ip directed-broadcast interface subcommand or the router has the IOS prior to 12.x, where directed broadcast was enabled by default, the router will encapsulate the packet in a frame with a destination MAC address of FF-FF-FF-FF-FF-FF.
I do not know which IOS version this question refers to, but it might be a very old question.
But anyway "not a valid IP address" is a wrong statement.
What do you think?
Routers drop broadcasts, end of story. This is the beauty of Cisco exams, the more you think, the more wrong you become!
sorry, i prefer to think more rather then less.
ip directed broadcast is not a simple broadcast which is dropped by the router
An IP directed broadcast is an IP packet whose destination address is a valid broadcast address for some IP subnet, but which originates from a node that is not itself part of that destination subnet.
ip directed-broadcast - command to enable the translation of a directed broadcast to physical broadcasts
IOS 10.0 This command was introduced.
IOS 12.0 The default behavior changed to directed broadcasts being dropped.
1. router from IOS 10.0 to 12.0 do not drop directed broadcast by default. Router after IOS 12.0 do not drop those broadcasts if enabled.
2. the statement "because it is not a valid IP address" is not correct what makes this answer wrong.
3. RT1 will encapsulate the packet in a frame with a destination MAC address of FF-FF-FF-FF-FF-FF is correct if RT1 has the IOS prior to 12 or was configured with IP directed broadcast command.
hope you can follow this logic.
Cisco Answer. A valid IP address is any IP address that is not a Network ID or a broadcast address.
Finding the Range of Valid IP Addresses in a subnet: One more than the subnet number - one less than the broadcast address.
Since address 192.168.1.255 is a broadcast address, B is the correct answer.
Please read Chapter 5 in the Cisco Press ICND1 book. It has some great "logic" you should read!
"It is better to remain silent and be thought a fool than to open one's mouth and remove all doubt."
-Abraham Lincoln
-Hondabuff
A+, Net+, Sec+, MECP, MCTS, CCENT, CCNA
(03-13-2012, 04:13 PM)Hondabuff link Wrote:Cisco Answer. A valid IP address is any IP address that is not a Network ID or a broadcast address.
Finding the Range of Valid IP Addresses in a subnet: One more than the subnet number - one less than the broadcast address.
Since address 192.168.1.255 is a broadcast address, B is the correct answer.
Please read Chapter 5 in the Cisco Press ICND1 book. It has some great "logic" you should read!
"It is better to remain silent and be thought a fool than to open one's mouth and remove all doubt."
-Abraham Lincoln
-Hondabuff
A+, Net+, Sec+, MECP, MCTS, CCENT, CCNA
Thank you, Hondabuff and special thanks to Abraham.
Your second extended answer is much better than the "end of story" kind of rhetoric. And my appeal to logic wasn't a personal attack, just a question if a valid
ip directed broadcast address falls under category of
IP address or is it rather a
non IP-address or
non valid ip address like 226.256.1.1
I have demonstrated before that in certain circumstances this broadcast would be forfarded into directly connected subnet with a destination MAC address FF-FF-FF-FF-FF-FF.
In some simlets here, for example QID:S27 you can find
no ip directed-broadcast in the configuration output what makes sence only if
ip directed broadcast was enabled by default on the router.
A.V.
MA in Philosophy
You Philosophy Majors always do think to much. Best way to handle the Cisco exams are to read the question and normally the driest/easiest question is normally correct. Wendell Odom even says "The Cisco Answer is always the correct Answer". I have seen a bunch of questions that are designed to make you overthink it and talk yourself out of the answer. The Cisco guys who write the exams are very good at what they do.