On Fri, Oct 31, 2014 at 6:47 AM, Marc Joliet <mar...@gmx.de> wrote: > Am Fri, 31 Oct 2014 07:52:54 +0100 > schrieb "J. Roeleveld" <jo...@antarean.org>: >> On Tuesday, October 28, 2014 07:31:56 PM Marc Joliet wrote: >> > >> > - I don't know whether we have an IP block or not; I suspect not. At the >> > very least, we didn't make special arrangements to try and get one. >> >> Then assume not. Most, if not all, ISPs charge extra for this. (If they even >> offer it) > > That's what I thought :) . >
Generally speaking you can't just attach a modem to your LAN and have it act as a DHCP server. Your ISP probably will assign you dynamic IPs, but they will not as a matter of policy assign you more than one unless you pay for them. IPv4 address space is in short supply these days. I'm using FIOS and in my case the "modem" is in a box in the basement and the ISP provides a router with the service. Whatever you plug into the "modem" will obtain a DHCP lease for one routable IP. If you do plug more than one device into the "modem" then the first device to get the IP is the only one that will get an IP - the modem won't hand out another unless it gets a DHCPRelease from the MAC that was issued the original lease or until that lease expires, or until you call up the ISP on the phone and get them to release it manually. Another design would be to issue a new IP anytime a device asks for one, but to silently cancel the lease of the last IP that was issued and drop packets using it. For a single device being plugged in that won't have any impact, and if for some reason you buy a new router and plug it in you don't have to worry about your old router still having a lease. This is less standards-compliant, but perhaps more clueless-friendly. In general, though, you really shouldn't be plugging your ISP's modem into anything but a router for general use. In fact, I have the router provided by my ISP configured as a bridge and running into another router (FIOS uses MoCA over coax in the standard install and I'm too lazy to run CatV and beg Verizon to reconfigure the modem to use the RJ45 connection instead). Note that if you use an ISP-provided router there is a good chance that they can essentially VPN into your LAN. The last time I called up Verizon over a cablecard issue they helpfully turned on DHCP on my router so that it started competing with my DHCP server, and then I was wondering why PXE was randomly failing. Now all they can do is disable bridge mode, which will break my external connection and be a fairly obvious point to troubleshoot. -- Rich
