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Why do I need to set bridge IP address outside the range of DHCP

Last response: in Wireless Networking
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August 27, 2012 2:59:52 PM

One common thing I have seen in many posts that I absolutely do not understand is the idea of assigning to a repeater bridge an IP address outside the range of the DHCP addresses assigned by the primary router. I have always set fixed IP addresses (with associated device names and MAC IDs) in the primary router for every device conneted to that router, and that has always worked for me.

What is the reason for suggesting that the repeater bridge should be in the same subnet but outside the range of pre-assigned IP addresses if those DHCP addresses are always fixed.??.

I now have a D-Link DIR-655 router and an old 802.11b Ethernet Adapter (DWL-810+) to which my HP printer is connected. That Ethernet Adapter and the printer both have assigned IP addresses in the DHCP address space and everything works fine. I am just trying to replace the old b-level Ethernet Adapter with an n-level box (e.g., a D-Link Dir-615 E3 flashed with DD-WRT) to connect my printer, as my Dir-655 router and all my computers are now n-level.
August 27, 2012 3:34:15 PM

The confusion stems from the fact that we’re often talking about reconfiguring routers as repeaters, and not using standalone, dedicated devices.

Routers don't permit the assignment of their own local IP address using DHCP. To have the router reference itself wouldn’t make much sense given the fact it’s determining the IP network, gateway IP, DNS server(s), and providing the DHCP server. IOW, the router isn’t normally a client to anything on the local network. Instead, it gets the ball rolling for everyone else by establishing its own local IP address (e.g., 192.168.1.1), and by extension, the network that implies (e.g., 192.168.1.x).

So now you decide to turn that router into a repeater. But again, it was never designed to assign itself a local IP address via DCHP (as I described above). And if you can’t use DHCP to assign the local IP address, then you certainly don’t want the static IP you do assign to conflict w/ any IP address that might be handed out by the primary router’s DHCP server to some other device.

Now in the case of commercial, standalone repeaters (and client bridges, printer servers, etc.), they *do* support DHCP because it’s part of their normal functionality to be clients.
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August 27, 2012 3:44:01 PM

P.S. I should also add, while most do, not every router’s DHCP server permits the binding of MAC addresses to fixed IP addresses. Some very low-end, budget models are so plain vanilla, it’s considered a luxury worth skipping. And of course, without static IP assignments, it gets very difficult to administer these infrastructure devices.
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August 27, 2012 4:43:07 PM

I'm sorry but I guess I don't quite understand -- or perhaps I was not clear in the first place.

My router -- a D-Link DIR-655 -- has the standard IP address of 192.168.1.1, with DHCP enabled. I have set the router's DHCP address range as 192.168.1.100 to 192.168.1.199. In the router's DHCP Configutation section, I have given every device that connects to my system a pre-assigned IP address in that range. I have also used MAC ID filtering in the router, although that is currently disabled while I am trying to set up a new DD-WRT Client Bridge.

So.. I am trying to replace my existing D-Link DWL-810+ Ethernet Adapter with a D-Link DIR-615 E3 configured as a Client Bridge to do exactly the same thing -- serve as a wireless connection for my Ethernet HP printer. The printer is one of the devices which has a pre-assigned IP address in the router (192.168.1.102) -- and it is this IP address that is used when I want to add that printer connection to a new laptop -- e.g., for a guest visiting with their own computer.

In the current configuration, the DWL-810+ Ethernet Adapter also has its own IP address -- 192.168.1.103, and that box has to be configured with the proper SSID, fixed channel and encryption keys to connect to the router. This configuration should presumably be the same for both the current Ethernet Adapter and its DD-WRT Client Bridge replacement. In my mind, they should be plug-compatible replacements for each other, and the only thing to ever connect to the Bridge is the one printer via a LAN port.

I have set up the DD-WRT Client Bridge according to the DD-WRT instructions -- but using 192.168.1.103 as the IP address -- and all looks fine when I connect the Bridge via one of the LAN ports to my laptop. I can see all of the proper DD-WRT configuration screens, and can also see that it is talking to the router -- the router's MAC ID shows up in the list of connections, and there is a good count of packets transferred between the Bridge and router. When I log into the router, I can also see that the Bridge has been connnected with the pre-assigned IP address.

If I set up the Bridge using a recommended address outside the router's DHCP range -- e.g., to 192.168.1.2, I can still see from both the Bridge and router sides that the Bridge is connected to the router, but the router only shows a connection via the MAC ID, with no assigned IP address.

And in either case -- setting up the Bridge with a fixed address inside or outside the router's DHCP range, I cannot wirelessly ping the Bridge or connect wirelessly to the Bridge from my computer the way I can with the current Ethernet Adapter. The only way I can connect to the Bridge is via Ethernet from my laptop -- even though the Bridge is somehow connected to the router.

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August 27, 2012 6:22:14 PM

Just about everything you described was perfectly correct. In fact, I have no idea what your original question has to do w/ your latest reply, because they're completely unrelated.

I suspect it’s because you drew the conclusion (erroneously) that your current problem was due to this issue of the static IP and DHCP, but it's not. It’s most likely a bug in DD-WRT having to do w/ that specific router, DIR-615 version E3.

http://www.tomshardware.com/forum/41764-43-setting-dir6...

You should read the whole thread to get the full context, but the crux of the problem can be found in the Best Answer post.
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August 27, 2012 8:09:34 PM

eibgrad said:
Just about everything you described was perfectly correct. In fact, I have no idea what your original question has to do w/ your latest reply, because they're completely unrelated.

I suspect it’s because you drew the conclusion (erroneously) that your current problem was due to this issue of the static IP and DHCP, but it's not. It’s most likely a bug in DD-WRT having to do w/ that specific router, DIR-615 version E3.

http://www.tomshardware.com/forum/41764-43-setting-dir6...

You should read the whole thread to get the full context, but the crux of the problem can be found in the Best Answer post.



In the end, the problem was with the firmware version I had used. I had previously followed several DD-WRT forums and came to the conclusion that 17201 was the best firmware version to use. But following your threads, it seems that the one which worked for other people was 14896. So... I downloaded that one, flashed the DIR-615 saying do not restore factory default settings -- just update firmware and leave settings alone. As soon as I did that, it worked perfectly just as I had it setup with my router -- literal plug-and-play replacement for the old Ethernet Adapter.

Again, I setup the Bridge with an address inside my router's DHCP assignment range, and entred the Bridge into my router's DHCP Configuration table.

THANK YOU AGAIN FOR YOUR HELP.!!!
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