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At my business I have set up a network to accommodate both wired and wireless connections. My setup includes a Motorola Surfboard SB6141 cable modem, an Ethernet switch, and two Airport Express routers. I recently had my old switch go bad (it was actually an old wireless router acting as a "dumb" switch), so I replaced it with an actual 5-port Linksys switch. Now I cannot get anything to play nicely.

If I shut everything off and then power up first the modem, then the switch, I have Internet connections on the wired PCs. If I turn on the Airports after that, the main Airport Express won't be given a valid IP by the modem's DHCP server and instead ends up with a 169.254.x.x address.

If I shut everything off and then power up first the modem, then the Airports and the switch with all PCs disconnected, the main Airport Express will grab a proper IP address but will refuse to connect to the Internet even still. Connecting the wired PCs after this results in them self-assigning invalid IP addresses.

The main Airport is in DHCP and NAT mode, while the second Airport is acting as a WiFi extender so it's in Bridge mode. Placing both routers in Bridge mode breaks the Internet connection via WiFi, but it doesn't make sense to me that the first router should work properly in DHCP/NAT mode since the modem is already running a DHCP server, and in my limited understanding it seems to me that this would either create conflict or at least a subnet on my network, which I don't want either. What, if anything, is wrong with my network setup, and how do I correct it?

migrated from serverfault.com Apr 29 '14 at 15:55

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  • Are you sure that your wireless router wasn't doing DHCP? I'm wondering if what changed is that your old wireless router was getting an IP address from your Surfboard, and then doing NAT/DHCP for your wired connections. Is that possible? It might be good if you describe in detail how things were plugged together when it was working. – msouth Apr 29 '14 at 0:51
  • Actually, that's exactly what was happening, now that you mention it. Setup was CABLE -- MODEM -- OLD ROUTER -- LAN (including Airports and wired network). Then I replaced the OLD ROUTER with NEW SWITCH, and it didn't occur to me that it was the router doing DHCP, and not the modem. I just checked one of my wired PCs and it's reporting no DHCP server present. – Nate Apr 29 '14 at 1:52
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It sounds to me like you are trying to setup the airports in front of or adjacent to your existing "old wireless router". Typically speaking most cable modem providers only provide a single WAN IP per modem (there are some instances where they might give you more than one, but I'll assume this isn't the case from your description).

What happened before is that odds are, your "old wireless router" as acting as the router between the cable modem and your LAN. It would be the only device receiving an IP from the cable provider and would default become the main default router. You airports would then become a downstream wireless router from there.

In your case, you need a device to be the front facing WAN interface. One of the airports will work fine. The difference is, on the second airport, you'd want to disable routing and use it as a wireless access point vs a wireless router. Since the main airport would be providing routing services to the cable modem, there is no reason to have the second airport do the same thing.

So you are right, the first router would handle DHCP/NAT and the secondary should just be a passthrough.

If your surfboard can do NAT, you might consider using that alternatively, but I wouldn't trust it to be as hardened as one of the apple devices. But I suspect that a few people might argue the other way for that as well.

  • While that seems to be what I overlooked, what I am confused by is the fact that when a device is plugged into the modem, it is assigned an internal IP. This would seem to indicate that the modem is capable of handling DHCP (and as a top-of-the-line modem, the Surfboard should!), but when I checked one of my wired PCs it told me there was no DHCP server present on the network. Reading other websites leads me to believe that the DHCP server only kicks in when the connection to my ISP is down, and otherwise it's simply passing through the public IP address. – Nate Apr 29 '14 at 2:18
  • Also, in the modem's configuration utility there are apparently no configurable options at all, it's all just information. So no checking for/configuring DHCP and/or NAT that way. Looks like I'll have to re-arrange the network a bit to compensate. – Nate Apr 29 '14 at 2:19
  • @Nate in Cable TV industry parlance, if it's a "cable modem", it's just a modem. If it can do DHCP and NAT, they call it a "cable gateway" or "eRouter", not just a modem. So a top-of-the-line "cable modem" might be faster than all the other cable modems, but it probably still won't do DHCP and NAT. – Spiff Apr 29 '14 at 21:34
  • @Nate, the 169.254.x.x address is generally a default self assigned address when DHCP doesn't answer. Windows does the same thing if you plug it into a network set it for DHCP and don't have a DHCP server. So that IP was expected. – MikeAWood Apr 30 '14 at 0:54
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How about something like this:

Plug the main Airport Express into the Surfboard (using the ethernet port on the Airport with the circular symbol).

There should be a specially marked port on the linksys called "uplink" or colored differently or identified somehow. Plug that into the other jack on the airport express. ("uplink" on linksys to the "regular double-arrow ethernet symbol" port on the main airport).

If there is a power-on sequence that matters (i.e. if it doesn't work to just randomly power them all up), I think it would be 1) Surfboard 2) main Airport (wait and let it get connected) 3) linksys. Plug the wired pcs into the other ports on the linksys and hopefully you are good to go.

My reasoning is that the Airport Express will get an ip from the Surfboard, and then the airport will do DHCP and NAT for everything else (which it sees through the Linksys).

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