ARP Question...

I was reading through Odoms INCD1 book and came across this question...

If PC3 were added to the LAN on the left, with the IP Address of 10.1.1.130/25, with the default gateway of 10.1.1.1, which of the following would be true?

A. If PC1 issued a ping 10.1.1.130 command, PC1 would use ARP to learn PC3's MAC address.
B. If PC3 issued a ping 10.1.1.10 command, PC3 would ARP trying to learn PC1's MAC address.
C. If PC1 issued a ping 10.1.13.1 command, PC1 would ARP trying to learn the MAC address of the 10.1.13.1.
D. If PC1 issued a ping 10.1.1.130 command, R1 would ARP trying to learn the MAC address of 10.1.1.130
Answer: A,D

According to the book PC3 would not ARP for PC1's MAC address if PC3 tried to ping PC1... (Option

. I'm not quite understanding the reasoning behind that because when I set the exact topology up in Packet tracer, PC3 would send out an ARP looking for PC1. Is this because I added a switch and connected PC3 and PC1 to it, and then the switch to the router, or am I way off base here??

Thanks for any help!

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Comments

Forsaken_GA

You didn't look close enough at the subnet masks.

PC1 is configured with a subnet mask of 255.255.255.0, which means that it thinks anything from 10.1.1.0 to 10.1.1.255 is on it's subnet. Hence, when it goes to ping PC3, it ARP's, because it thinks it's local

PC3 should be configured with a subnet mask of 255.255.255.128, which means it thinks anything from 10.1.1.128 to 10.1.1.255 is on it's subnet. So when it goes to ping 10.1.1.10, it sees that it's not on the same subnet, no ARP request. That's why B and C aren't valid, they're not on the same subnets as their ping targets (from their perspective), so no ARP.

simonmoon

PC3 has been configured on the /25 subnet, so it doesn't think 10.1.1.10 is on it's network. It would instead ARP for it's default gateway.

osJoe

I see, thank you! I didn't even think about that because I have this bad habit of assuming all the PC's on a LAN should be in the same subnet.

angel.oa

osJoe wrote: »

I see, thank you! I didn't even think about that because I have this bad habit of assuming all the PC's on a LAN should be in the same subnet.

bad habit

MrRyte

osJoe wrote: »

I see, thank you! I didn't even think about that because I have this bad habit of assuming all the PC's on a LAN should be in the same subnet.

ALWAYS be extra wary of the subnet mask and default gateway of the network that you're working on (especially on labs and exams) They are so easily overlooked but can cause major headaches.....