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For work, I have designed a hardware 'box' (actually an industrial control gizmo) which has an embedded web server for the purpose of configuration (much like a regular domestic Internet router has a web server for similar purposes). It's usually used with a direct connection to a laptop, so that a regular browser (IE8, FF, etc.) may be used to configure the box.

The problem that has recently come to light is that if the 'box' is connected via wired Ethernet to certain Netgear or Linksys WiFi switch/router units and then an attempt is made to access the box via the switch using a WiFi connection to the switch from the laptop, the laptop browser is unable to connect to the box (resulting in a typical 'server not found' error). However, if the laptop is connected via wired connection to the switch, the box can be accessed just fine. It's almost as though the problem is specific to just WiFi.

To clarify with some examples:

  • Linksys or Netgear wireless router/switch(/modem) configured with IP of 192.168.0.5.

  • My custom hardware with its embedded web server configured with static IP of 192.168.0.100 and has a wired Ethernet connection to the Linksys / Netgear unit.

  • PC obtains IP via DHCP from the Linksys/Netgear unit; let's say it's at 192.168.0.200.

  • If the PC is connected via wired connection, the browser can access the box at 192.168.0.100 just fine.

  • If PC connected to the router over WiFi, then it can't access the box. But it can happily access the router's own web server at 192.168.0.5.

  • Even when the PC's browser won't connect when WiFi is in use, I'm able to successfully ping the hardware box from the PC.

  • From what I can recall, when the connection doesn't work over WiFi, I don't seen an entry for 192.168.0.100 with arp -a. When the connection is working (PC has wired connection to router), arp -a shows me an entry for 192.168.0.100.

I have been currently trying to investigate this today with a Linksys WRT54G. At first, I had the problem as described above. Later, after much messing about, it somehow resolved itself. The only procedure I recall doing immediately before it magically started working over WiFi was a series of successful ping tests from the router itself to the hardware box.

This problem has been reported with several Netgear / Linksys routers, though it will be a bit of time before I can determine the model numbers.

Any help would be much appreciated. Please let me know if there's any further information or logs or tables I could provide.

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2 Answers

It sounds like you have multiple routers and multiple networks. If the two machines aren't on the same network, it won't work. Since each machine sees the other machine's IP address as inside its local network IP range, it assumes it's on the same network. If it's not, it won't work.

If you have two networks both numbered 192.168.0.0/24, a machine one one network won't be able to reach a machine on the other network. Your ping is reaching a different machine on the same network with the same IP address as the machine you were trying to to reach.

When the wireless system happens to connect to the same network, then it all works.

You can confirm this theory as follows: Ping the default gateway on both machines. Then find the default gateway in the ARP table. See if the MAC addresses are the same. If they're not, that's the issue. They're using different routers.

Update: I like Tony Roth's theory. Maybe you have a web proxy configured that's outside of (or not allowing connections to) the local network.

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Hi David, many thanks for your answer. However I can confirm that this is not an issue with multiple networks. The 'box' (i.e. my product) and PC are all on the same subnet (192.168.0.x) and connected via the same router. When box and PC connected via wired Ethernet to the router, then PC sees the box no problem. When PC is actually connected via WiFi to the router - and it is definitely connected to the correct WiFi AP - the PC's browser is often no longer able to see the box, even though it can still be pinged, and the browser can still access the AP's own internal web configuration pages. –  Trevor Page Apr 15 '12 at 12:17
    
Having done much searching on this earlier, the closest router/AP feature I could find that could possibly cause this would be WiFi client isolation. But, isolation is turned off on this Linksys router by default. Furthermore, when my PC (connected via WiFi) was suddenly able to see the box, turning on the isolation feature didn't prevent the PC from seeing the box. –  Trevor Page Apr 15 '12 at 12:19
    
Then you might want to describe your network topology in some detail. How many access points do you have? How many routers? How are they hooked up? (And did you double check that the default gateway's MAC address is the same?) –  David Schwartz Apr 15 '12 at 12:19
    
Literally just one Linksys router/AP (.0.5). My black box (.0.100) connected to router/AP via wire. PC (.0.200 DHCP) connected to router/AP via wire: browser can see the black box. PC connected to AP via WiFi instead of wired connection: for a long time, browser unable to see box, though I can ping the box from cmd. Later, browser can suddenly see the box even when PC connected over WiFi. However, a colleague near London (too far for me to visit!) is having exactly same issue as described, but browser never sees box via WiFi to router; only sees it when PC connected to router via wire. –  Trevor Page Apr 15 '12 at 12:25
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try to telnet to the device on port 80 to see if its a proxy issue. –  tony roth Apr 15 '12 at 17:31
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up vote 1 down vote accepted

This is quite embarrassing, but it turned out that it was due to the MAC address that I had assigned to the box. Being an R&D PCB without production assigned MAC, I just put a dummy MAC address in. What I had put in had the multicast bit set (if I remember right - this is going back months now). I changed it to a more standard MAC address and all was fine.

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