Subnets - Stumbled into a "Why?" moment

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Russell
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Subnets - Stumbled into a "Why?" moment

Postby Russell » Mon Jul 13, 2009 11:57 pm

So, I have a simple wireless network setup at home on my DSL modem in the following fashion:


[Internet]
|
|
DSL Modem
Internal IP: 192.168.1.254
Subnet: 255.255.0.0
|
|
Wireless Router
192.168.2.1
DHCP Enabled
192.168.2.100 - 192.168.2.150
Subnet: 255.255.255.0



All wireless clients are handed an IP from 2.100 to 2.150, and a subnet of 255.255.255.0.

Here's my "Why is this working?" moment:

Even though my machine is handed a subnet of 255.255.255.0, I can still browse to the modem's internal IP of 192.168.1.254 and view its status and information pages. That's great, I'm glad it's working, but WHY is it working? According to the subnet that this machine has, it shouldn't be able to reach any of the 1.X IP addresses.

To prove it, here's my ipconfig:

C:\Users\Russell>ipconfig

Windows IP Configuration


Wireless LAN adapter Wireless Network Connection:

Connection-specific DNS Suffix . :
Link-local IPv6 Address . . . . . : fe80::89d9:e37:4754:8cb6%9
IPv4 Address. . . . . . . . . . . : 192.168.2.100
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.2.1



What's the deal? :???:
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nitrogen
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Re: Subnets - Stumbled into a "Why?" moment

Postby nitrogen » Tue Jul 14, 2009 12:15 am

Guessing your gateway at 2.1 is forwarding to the cablemodem at 1.1?

Betting the wireless router has a default gateway of 1.1.
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Russell
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Re: Subnets - Stumbled into a "Why?" moment

Postby Russell » Tue Jul 14, 2009 12:57 am

Actually, it's a little weirder than that. Here is the status page of the wireless router.

IP Address: 69.151.205.240
Subnet Mask: 255.255.0.0
Default Gateway: 69.151.207.254


That IP address is the external IP address of the DSL modem (The DSL modem is set to hand its IP off to the router). As you can see, the gateway is a different IP than the DSL modem so I guess that's just an AT&T thing.


So, is the reason why I am able to access my modem's status page because it is outside of the LAN, and therefor the router is handling it?

I suppose that makes sense. I pulled my laptop out and gave it a 1.x IP, and my desktop couldn't reach it unless both machines had a subnet of 255.255.0.0.

Interesting, didn't think about that. :thumbs2:
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PetrucciFan
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Re: Subnets - Stumbled into a "Why?" moment

Postby PetrucciFan » Thu Jul 16, 2009 9:04 am

Your modem is probably set to "bridge" mode. In bridge mode, your modem does not actually establish the PPP link; it simply passes through the data packets to another device (in your case, a PPP cable router, probably a Linksys WRT54g or similar). In your last post, you said that your DSL modem is set to hand off its IP to the router, which is not technically true (the modem does not have an external IP.....it is assigned directly to your router). That is why your modem has a 192.168.x.x address while your router shows an actual public address.

There should be two automatic routes on your route table, one for 0.0.0.0 (anything on the internet), and one for the DSL modem. Since it is a router, it can talk between subnets.

So your question about the modem being outside of the LAN is true.
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Topic author
Russell
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Posts: 2774
Joined: Sat May 20, 2006 12:46 pm

Re: Subnets - Stumbled into a "Why?" moment

Postby Russell » Thu Jul 16, 2009 10:39 am

Actually, this modem is interesting. You can either set it to where it assigns an internal IP address to the router OR the external IP. AT&T likes to make things more difficult than they should be :mrgreen:

Of course I have it set to assign the external IP for simplicity sake, and the router is set to Gateway mode.

I hadn't ever thought about the DSL modem itself being outside the LAN. Thanks for all your replies :thumbs2:
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lfinsr
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Re: Subnets - Stumbled into a "Why?" moment

Postby lfinsr » Tue Oct 13, 2009 9:51 pm

Russell,

I know this is an old thread but if you're still interested in know how this works let me know and I'll spend the time typing up an explanation. If you've already got it figured out I won't waste the time...

Larry
My guns won't be illegal, they'll be undocumented. :thumbs2:


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