9 Replies Latest reply: Jun 1, 2007 8:44 PM by jph_california
jph_california Level 1 Level 1 (0 points)
Okay, here's a good one for you all: is there any way to control/specify from which DHCP server an Airport Express will get its IP address? Here's the problem: I'm on a shared Ethernet network I don't control, which has a DHCP server that assigns addresses, and through which I can access the Internet. If I plug my iMac or PowerBook directly in to the network using the Ethernet cable, the computers request and receive an IP address (DHCP lease) from the correct DHCP server. BUT, when I connect the Airport Express in to the same Ethernet network, it receives an IP address from another DHCP server on the network--a "rogue" server installed by someone on the network who doesn't know what they're doing, and that gives out IP addresses for the wrong network/subnet.

In short: plug my iMac in to the Ethernet network, and it gets its IP address from the right DHCP server. Disconnect that cable and plug it in to the Airport Express instead, and the AE gets an IP address from a different DHCP server.

I've tried restarting and resetting the AE (back to factory defaults even), but it always gets its IP from the wrong server. WHY? And, can I control this on the AE somehow?



iMac 17" Intel   Mac OS X (10.4.5)   pBook G4 12", iBook G3 12"
  • 1. Re: Airport Express gets IP from wrong DHCP server
    Frank Canzolino Level 7 Level 7 (25,875 points)
    Disable Distribute DHCP on the AX.
  • 2. Re: Airport Express gets IP from wrong DHCP server
    jph_california Level 1 Level 1 (0 points)
    Frank, thanks for your reply. I assume you mean, disable "distribute IP addresses" in the Network settings on the AX. (I don't see anything called "Distribute DHCP" in the AX settings.) In any case, disabling "Distribute IP addresses" on the AX does not resolve the problem. I've already tried that.

    I've also tried giving the AX a manual IP address, restarting, and then returning to a DHCP-assigned IP address. The AX always grabs the same (wrong) IP address, from the same (wrong) DHCP server.
  • 3. Re: Airport Express gets IP from wrong DHCP server
    Frank Canzolino Level 7 Level 7 (25,875 points)
    Can you give some examples of the right and the wrong addresses you are seeeing?
  • 4. Re: Airport Express gets IP from wrong DHCP server
    jph_california Level 1 Level 1 (0 points)
    Sure. By "right" address, I mean an address assigned by the proper DHCP server on the network--the one set up by the network administrators, which assigns addresses that then in turn allow access to routers, DNS, Internet, etc. These addresses are typically in the 128.195.89.xxx range (255.255.255.0 sub) for the part of the network I am on--purely internal network addresses, of course.

    By "wrong" address, I mean the AX gets its IP address from a "rogue" DHCP server--one incorrectly set up on the network by someone other than a network admin, just somebody plugging in a router/DHCP server to the single Ethernet jack in their office/room, probably intending to use it to create their own "home" network so they can have multiple machines connected using that one Ethernet wall jack they've got--and not knowing what they're doing, they set the thing up to assign IP addresses via DHCP to the network at large. These addresses are in the range 192.168.0.xxx (again, 255.255.255.0 sub).

    Just to repeat the problem, the weird thing is, if I plug my iMac or pBook in to the wall jack, thus connecting to the network that has both the right DHCP server and the "rogue" DHCP server on it, the iMac or the pBook always get their IP address from the right DHCP server. When I plug the AX in to the same jack, it always gets its IP from the wrong, "rogue" DHCP server.

    The network admin folks here are no help--they say that they only support problems with computer connections, not router connections, so they won't troubleshoot the AX problem or block the rogue DHCP server on the network, since it's apparently not assigning IP addresses to any computers, only my AX (and possibly other routers, though I've no way of knowing).

    I am wondering if the rogue DHCP server is maybe another Apple router (Airport Extreme Base or something?), and if the AX has some kind of preference for other Apple routers built-in, so that given a choice, it gets its IP from the Apple product?? Just a wild guess.
  • 5. Re: Airport Express gets IP from wrong DHCP server
    Alan Somers Level 6 Level 6 (9,345 points)
    The only way that multiple DHCP servers will work reliably on a single LAN is if they distribute mutually-exclusive addresses. However, it's then a crap shoot as to which DHCP server will serve up a client's address. There is no way that I know of to target a particular DHCP server and its typically a bad idea to have multiple DHCP servers on a single LAN.
  • 6. Re: Airport Express gets IP from wrong DHCP server
    jph_california Level 1 Level 1 (0 points)
    Alan, of course you are right on both counts--it only works when they distribute mutually exclusive IPs, and it's a bad idea to have more than one on a single LAN segment in most cases. However, this isn't something I can control--the network segment I am on is a large one, shared by many users at the university. If someone wants to plug a DHCP server into the network, they can. Thus, my need to somehow tell my Airport Express to ignore these stupid machines...
  • 7. Re: Airport Express gets IP from wrong DHCP server
    Alan Somers Level 6 Level 6 (9,345 points)
    Can they assign you a fixed address? That would fix the problem by allowing you to disable DHCP completely on the Express.
  • 8. Re: Airport Express gets IP from wrong DHCP server
    Frank Canzolino Level 7 Level 7 (25,875 points)
    Alan is absolutely spot on. The network administrators should also track down the clown with the conflicting router and shoot him dead.
  • 9. Re: Airport Express gets IP from wrong DHCP server
    jph_california Level 1 Level 1 (0 points)
    I'm closing this by marking it as "answered"--although there were no solutions to the problem of telling the AX which DHCP server to use. However, the last poster was correct--I managed to convince the network support folks to track down the offending router/user and shoot it/them. (Well, hopefully just "it.")