My question for Admin.

Hello Chris,

I'm a newbie in networking and I've just started doing self-study on CCNA recently. So please excuse me if you find my following question sounds stupid [img]images/smiles/icon_wink.gif[/img]

I have a query about your experiment "Testing communications between subnets" (see www.firewall.cx/index.php?c=ip-subnetting-routing ). I don't quite understand why your workstation could not communicate with the other 2 stations after you had modified the subnet masks. Well, recently I read about this ARP (address resolution protocol) thing in a guide. It says that if a host is configured with subnet mask and default gateway, it is able to tell whether the IP address of a destination is locally or remotely located. If local, it would send a ARP request for the IP address of destination. If remote, then it sends an ARP requesting for the MAC address of the default gateway. Applying this theory in your case, I suppose your workstation should send an ARP broadcast requesting for the MAC address of the Linux Firewall since it is already properly configured?

I would appreciate very much if you could explain more details about this particular example.

Many thanks in advance and keep up the good work.

Dear steelben,

Your observation is totaly correct, however your missing one small detail that will make the difference [img]images/smiles/icon_smile.gif[/img]

Refering back to the same example, when I changed my subnet mask to 255.255.255.224 i split the one large 'logical' network (255.255.255.0) into eight smaller networks. Keeping this inmind, and the fact that the IP address my workstation had meant that it was on a different logical network from the rest of the hosts, it wouldnt be able to contact them unless it had a gateway that was in the same network.

If you notice my gateway was still confiogured to 192.168.0.1. This IP address, after the change i did, was not in the same network as my pc. So, my PC was part of the 192.168.0.32 network while the Linux and Netware machine were part of the 192.168.0.0 network. The only way my PC can contact any of the two hosts in the 192.168.0.0 network is via a Gateway. This gateway MUST be part of the 192.168.0.32 network, otherwise my workstation cannot talk to it.

This is shown clearly in the next diagram where I gave the Netware machine an IP address of 192.168.0.36 and modified my workstation's gateway to match this IP address. This way my workstation sends the packet to the netwate machine (my new gateway) hoping that it would know how to get to the network that the linux machine is part of.

I really liked your question as it shows a true understanding of whats happening... kinda reminds me of myself when I was learning all this cool stuff.

I hope the above helped. If you have any futher queries, let me know.

Chris P.