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DHCP problems

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by rebootd

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I am running into a recurring problem: very often when I connect DHCP cablemodem clients to the internet, (a very busy DHCP server, btw)- I cannot get an IP from it. I can take one computer, put it on the same connection, and get an IP immediately. Then I take that one off, put the other one on, and ipconfig/release and /renew, reboot, reboot the modem, check network settings for the fifteenth time, etc etc and still get nothing but zeros. The error message I get is that the DHCP server is unavailable, or that it timed out. Then, I can try the other computer again: even ipconfig/release all, then /renew, and it gets an IP immediately again. Can someone tell me if there is a hidden DHCP or network setting somewhere, maybe in the registry, whatever, that I do not know about?? This has happened with all desktop Windows OS's, although the scenario I just described happens regularly with both machines having the same OS at the same time. It *seems* as if it happens more often with Windows XP, although that may be just because more people own XP now. The NIC's are always working correctly when this happens: cables and hardware are always the first thing I check, and I can ping the loopback address just fine. Somebody please help?

15 replies

by samlowrey

I may have an answer - I don't know if it is the solution because I don't know what you are trying to set up here. I do know that the cable modems in my area remember the MAC address they have assigned an IP address too. This was to prevent multiple IP addresses being assigned (I would think there was a better solution). Anyway, you have to unplug the modem for over a minute to get it to clear. So, what may be happening is an IP is being given to one MAC address on the first computer, then it refuses to give out another (assuming you are only paying for one). Check to see how many they are paying for.

by rebootd

When I reboot a modem I usually leave it disconnected for a minute or so already - you are correct that the session "remembers" the MAC address. However, this is DHCP so they are not paying by the IP. I am wondering if maybe this might have something to do with NIC duplex settings. Anyone?

by mrjay67

It doesnt matter that its DHCP. Its not about per address. The fact that the session remembers the MAC is the concern. Since the cable company ties the leased IP address to the MAC it still gets stuck that way. I used to have the same problem tillI just got a Linksys Router and it handles all that for me. To answer you question about duplexing you might want to leave it on auto or half duplex. Alot of ISPs(Cable/DSL) do not support Full Duplex. The way around that is to use a router/hub like the linksys(any brand works just an example). Anyway the issue sounds more like the IP being tied to the MAC and not releasing. There were times I had to call and get the cable company to try and reset things on their end but that gets nightmarish. For the time you spend talking to 1st levle support you can leave the modem off/unplugged and resolve the issue yourself.

If your speed and duplex settings are to high you will have problems as well but then you wouldnt get any address or direct conectivity to the cable Co.

From my own experiances this is the likely scenario. If its neccessary to use both PCs get a Broadband router/hub and this will solve those kind of headaches.

Jason

by mrjay67

Something you can try to get around things. If there is a spot on the NIC settings for network or hardware address, try putting in the MAC of the one that gets the IP immediatly. I havent tried this but thats what the space is supposed to be for.I know on my linksys there is a spot for spoofing the MAC of your NIC if you have this kind of issue. Try that as a work around. Hope this helps.

by mrjay67

I can understand your problem with customers not wanting to spend the money on extra equipment. The unfortunate fact that seems to arise with broadband Cos is that they try to limit the # of PCs we use. Especially when its residential svc its usually restricted to 1 PC. If these are companies they should invest a small amount in some kind of broadband router to ease this issue. The small 4ports usually range about $50-$80 online. I would imagine they range $80-$100 in a typical retail store. I realize, though, this may not be an option.

Another possibility is once the IP is obtained turn it into a static address on each PC. This should take care of the modem trying to see if you belong. The problem seems to lie more with the modems as that appears to be where the MAC info is stored since the modems are the pieces neededing to be reset.

I understand to a point(businesswise) the cable companies trying this tactic but its a pain when switching PCs in and out.
I have to concede that there may be other issues going on as well that im missing here but the main issue is the modems storing the MAC. I suspect that the Cable company controls this setting even if it could be turned off at the modem it would probally just getreset again.
As I said previously you could try to spoof the cached MAC on the NIC. Thats the only 2 workarounds I can think of that dont use extra equipment.

I have experienced the same issues and problem is the IP-MAC caching. Its not just keeping the IP for that MAC, its saying that only that MAC can get an IP period and why you have to go through the reset process. If you put the PC on a standard network with DHCP it will release and renew as long as there are adresses available so you are correct in that assumption. But this is a different animal with controlled settings that keep PCs from getting address at times.

Try the spoofing it should help.

by rebootd

Please see comments from 11/1 - I think I may have posted that one incorrectly.

by rebootd

Unfortunately, I can't just tell a customer to go buy networking equipment to solve their problem - and anyway a computer should be able to get an IP by itself anyway. The MAC address of the modem is not the problem - the modem gets an IP no problem, and it's address is provisioned by the ISP - which is how we know who to bill. The number of computers on a given modem is not an issue - anyone can network their computers if they want to and the modem is capped to a certain bandwidth anyway. It is the computer's (or router, if networked) MAC info that I suspect is being cached some way...as if once the server ties a computer MAC address to a cablemodem MAC address, it holds on to them both until either the lease expires or it is told different. However, this theory goes all to hell because sometimes I can plug into a customer's modem having never used it before, and the server gives me an IP instantly. Also, sometimes if I can get my own computer online, and not a customer's, I can reset the modem from our server and quickly yank out the CAT5 from my computer - then plug in the customer's computer and get an IP for them that way - however, if starting out I cannot get either theirs or mine online - and on our server there is no computer IP populating that field, this process then does not work. And, the company has certain brands it will let me use, and Linksys isn't one of them :\

by rebootd

No, it is the modem's bandwidth that is capped - it does not matter how many PC's the customer wants to use. They can put as many as they want on there, it doesnt change the amount of bandwidth assigned to them - and we do sell networking equipment if they want it - my point is that a PC should be able to get a DHCP address by itself - many people only have one PC and only want one PC. Also, you cannot "turn it into a static IP" unless you want to pay for a static IP. The way DHCP works is witha timed lease - it renews itself after a set period of time - if you set it to static, it will not work. Remember, I work for the ISP itself - I can reset from the server even, and still certain computers refuse to gain an IP. Why this is, is what Iam trying to figure out.

by mrjay67

I really dont have much else to offer. The suggestions I have offered should get you by, but then it all may depend on the cable company(or the modem they use). I do know that it is possible to keep dynamic addresses as static. Once you get an address pulled down you can configure the PC staticly with that IP and DNS info. It should keep unless the the DHCP server gets reset, IP scope/scheme changes or the PC is off after the lease expires. Really the only one that will cause problems is the IP scope/scheme change. The other two have to be in conjunction with each other and with another PC getting the address before the other. That would be the easiest way for me to solve the problem. Now since you said you work for the ISP or something to that effect you probally aren't allowed to do this. I understand that. The next thing to try is using a MAC address that gets the IP quickly. Spoof it in the NIC properties. Not all NICs have the options. but most good ones have a hardwareor network address spot that you should be able to try this at. Those are the two easiest ways around it without buying more equipment.

Now if the problem is occuring with the same PC(s), and not because another one(or NIC) was used, then there is possibly another issue at hand. Since the issue you described is occuring on different users/clients PCs it is likely not the fault of the PCs. This is more the result of how the cable(and maybe even DSL) companies have the network setup. I'll give to the possibility of it being a combonation of the 2 but still lies more with the Cable Cos. Because, the same PC would, like you said, release and renew all day long. The thing is it will renew back to the same IP if it isnt taken yet.Im just repeating myself and I really dont know what else to explain here. If there is a solution to this other than my work arounds Id like to hear it.

Jason

by mrjay67

When I was saying this: "Because, the same PC would, like you said, release and renew all day long. The thing is it will renew back to the same IP if it isnt taken yet." I was refering to a standard network setup(obviously using DHCP). Sometimes mymind thinks of things faster than I can keep up typing or such.

by rebootd

Yes, I do work for the ISP. And no, I cannot give a customer a static IP when they are not paying for one, nor do I see a reason to. Did you really read the thread on http://www.experts-exchange.com/Networking/Q_20386107.html ? This problem exists in only a few clients: The DHCP server has no problem with the thousands of other DHCP requests. It is pretty clear to me that this is a client problem, not a server problem. For a given modem, when I clear old MAC info from the server and subsequently my laptop can get an IP now off that modem, and then clear it again and subsequently the customer's computer still cannot get an IP, then there is something wrong with the customer's config somewhere...this same scenario has occurred with a third computer (usually the customer's second computer) being also able to get an IP, (after clearing), but the "problem" computer again will not. (Again, no justification for a static IP at all.) Discover, Offer, Request and ACK (etc) are working just fine for all the other clients out there, so why not these specific problem clients?

by tlath1972

i was wondering if you are unplugging the other computer and then plugging the second computer in. You should do a ipconfig /release or do a shut down of the first computer before you unplug it from the modem. Another alternative is to set up a proxy to share the internet connection. One computer brings in the connection and all the other computers connect to that one.

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