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I'm studying for my Network+ Exam. I'm going through some practice questions, and this one has uber-stomped me in the brain. I've been going through my textbook too, and can't find a concrete answer, so I'm turning here for help.

Here it goes:

Jeff, an administrator, has just finished installing a new switch and connected two servers with IPs of 192.168.105.20 and .30. The servers are able to communicate with each other, but are unable to reach the Internet. Jeff sees the following information in the switch configuration: interface VLAN 105 IP address 192.168.105.5 255.255.255.0 Jeff is able to ping the router at 192.168.105.1 from the switch. Which of the following is the MOST likely cause of the problem? Select one:

a. Jeff used a crossover cable to connect the switch to the gateway. b. The server is missing default-gateway information. c. A routing loop has occurred. d. The subnet mask is incorrect.

I know the right answer, but I can't put together why. Would someone be so kind as to explain?

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If a host knows, based on its own IP address and mask, that a destination IP address is not on its subnet, it will send the packet to the configured gateway's IP address.

By the way, masks and gateways are the first things to check when working in the real world. You may often find that real subnets are not a neat as using /24 everywhere, and people often get the mask or gateway wrong. An incorrect or missing gateway could cause a host not to be able to get off the subnet, and a bad mask could cause a host to be unable to reach a particular subnet encompassed by an incorrect mask or not reach some host on its own subnet.

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The answer is B - its clear that the machines can all speak to each other, however they do not know how to send the information on through the router - ie the machines do not know the router is the gateway, so they have nowhere to send the packets for forwarding.

  • Damnit Jim. You're right. I got it now, thank you. The answer was staring me in the face. I think it was the wording of the question that got me. I was looking for an answer through the IP addresses and not in the wording of the question itself. – MIchael S Oct 24 '15 at 2:46

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