DHCP gone bad

[cellio]

Once upon a time we bought a Linksys router and configured it to hand out IP addresses dynamically. I forget why, but we specified a range of 100-150 for the final byte. (The router itself is 1. Practically speaking we'd never need as many as 50. These might have been defaults; I don't remember.) What this is supposed to mean is that it hands out addresses in order as needed, starting with 100, and if you ever have more than 50 machines on the network you have a problem.

This chugged along fine for a while until it started handing out out-of-range numbers. (No, we have never had so many machines that we exhausted the set range.) We couldn't stop it from giving my machine 192.168.1.2; when it did that my machine couldn't see the internet (presumably because this was out of range). We assigned fixed addresses (in range) to all the resident machines and carried on. I forgot about this until we switched DSL providers recently and found that my network settings were still referring to the old provider. (Once you specify IP address, you also end up specifying DNS servers.) Ok, back to DHCP. We replaced the router a year or two ago, so for all I knew this wasn't even an issue any more.

This morning I couldn't connect to the internet (after a reboot). After the usual diagnostics and quick fixes, I got around to looking at ipconfig. My IP address was, once again, 192.168.1.2. WTF? So this time I decided to change the router; I told it to start handing out addresses with 2 instead of with 100. That didn't fix it. So, finally, I assigned my machine a specific IP address, just like we did before, and it worked.

I still have no idea why this happens. I have a workaround, but the mystery still bugs me.

Comments

Got a musical favor to ask you

[kmelion.livejournal.com]

I'm putting together a slide show of photos Zach and I took of our trip to Belvoir Fortress and I'd like some music to go with it.

What would you suggest and would you possibly have something I can use?

Re: Got a musical favor to ask you

[cellio]

Can you tell me more about the fortress? When does it date from? Is it associated with any particularly-significant historical events?

Re: Got a musical favor to ask you

[kmelion.livejournal.com]

Here's some info:
From Wikipedia
Israel Tourism site

[grouchyoldcoot.livejournal.com]

This is a strange idea, but if your network is wireless, is it possible you temporarily connected to the open wireless network of one of your neighbors? I've had that happen; they have cheap routers supplied by their phone companies that are quite willing to grant me DHCP addresses. They don't use the same numbering rules as my router, and if I have a connection failure my machine can end up with my router's MAC address but a DHCP address from my neighbor. I think.

[cellio]

Good idea, but alas, no wireless here. We built our network during the dark ages. :-) (Wireless would be handy someday for laptops, but since all machines already here have wired connections, it hasn't been a high priority.)

[gregbo.livejournal.com]

Odd. DHCP leases addresses, so if the router is handing out out-of-range addresses, either there's a bug or some leases are not expiring.

I have a wireless Linksys router that issues one day leases. (I don't usually leave computers on that I don't use, so leases are renewed at reboot time.) The next time you reboot the system that you've had to give a static IP to, you might reenable DHCP and check to see what the router is handing you and how long the lease is.

Even if the router is giving out 192.168.1.2, in theory you should still have Internet connectivity as long as you are using subnet masks that are consistent with a router with a LAN network address of 192.168.1/24 (network 192.168.1.0, subnet mask 255.255.255.0).

grouchyoldcoot's idea was interesting. I'll have to file that away as a possible interview question. This sort of thing might be the difference between me getting and not getting a job.

BTW, tsennyipa is a DHCP guru.

[cellio]

Thanks for the advice. I hadn't thought to check for leases not expiring.

(We have three computers that stay on pretty much all the time, and a couple of transient laptops. I suppose laptops + unexpired leases could cause this problem.)

Even if the router is giving out 192.168.1.2, in theory you should still have Internet connectivity as long as you are using subnet masks that are consistent with a router with a LAN network address of 192.168.1/24 (network 192.168.1.0, subnet mask 255.255.255.0).

Hmm. Ok, I'm stumped then. I thought the router was being "smart" and only letting traffic through on in-range addresses; if that's not it then something weird is going on.