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PostPosted: Thu May 23, 2019 1:36 pm 
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My website security is tied to my ip address, so when I shut down my workstation due to lightning storms, I try to keep my ip address from changing. Thus, I need to leave the device that stores my ip address powered-up. I'm under the impression, regardless of which device retains the ip address, that I cannot disconnect my modem from the network (although I do disconnect the Ethernet cable between the router and computer). But I can't remember if the modem or router stores the ip address. I seem to believe it's the router. Can someone please confirm?

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PostPosted: Thu May 23, 2019 1:42 pm 
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Cable modem? DSL modem? The answer will vary depending on what type of internet connection you have and, more specifically, the ISP you have. There are broad strokes for particular types of connections but not all ISPs do the same thing.


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PostPosted: Thu May 23, 2019 2:14 pm 
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Aha. This would be a cable modem made to spec for Cablevision (Optimum).

Thanks, MB.

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PostPosted: Thu May 23, 2019 2:22 pm 
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I think you can power them all down, but you have to poer them up in the same order.

The Modem should be handing out IPs as requested by order of devices connecting... somebody correct me if I misunderstand.


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PostPosted: Thu May 23, 2019 5:45 pm 
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Okay. I'm not familiar with Cablevision, but if it's like most US cable modem ISPs here's the deal.

Your IP address should be assigned by a DHCP request to the ISP's servers. Those determine which systems get which IP. If the "system" assigned that IP is down when the DHCP lease expires, you're unlikely to get that same IP afterwards.

US cable modems normally run in bridge mode and bond with the first MAC address it communicates with over ethernet. Only that MAC address is allowed to talk to the cable modem from then on, barring you power cycling the modem first. This "feature" is why most routers support MAC cloning on their WAN port.

Now if your cable modem includes WiFi, even if WiFi is turned off, it's likely running in router mode. In router mode if you plug a system into the modem it'll get a NAT address (10.x.x.x, 172.16.x.x, 192.168.x.x). In router mode it doesn't care about MAC addresses of clients. But there's a caveat to this. When a bridged modem is first booting it will normally hand out a 192.168.x.x address after the modem has booted but before it's finished connecting to your ISP - after it connects to your ISP the device will get reassigned the public IP address. So if your modem has been running for a while, you plug a system in and you get a NAT address assigned to it, then it's running in router mode.

So, basically, if you have a modem running in bridge mode you need to keep both the modem & router running to hold onto your public IP address.

If your modem is running in router mode, only the modem has to be turned on, because it's the one with the public IP address.

There is some variation here, in particular some ISPs maintain their DHCP leases for crazy long periods of time and even after power cycling you can't change the device behind the modem because the ISP won't let another MAC address connect until the first device's DHCP lease has expired. If Cablevision is one of those, you're somewhat safer in turning your router off, but it's a bit of the luck of the draw - will your router be off when the lease expires? Safest is to leave it on. Personally I have a UPS just for my modem & router so those two can peacefully coexist on the same lifetime. OTOH it's not like they draw a lot of power so a little goes a long way.

This all goes out the window if Cablevision requires PPPoE, which would be really weird for a cable company, but not unheard of in the US. With PPPoE you'd need to have configured your router with a username & password given to you by the ISP and have put its WAN port in PPPoE mode. Kind of memorable for me since it's so uncommon (PPPoE is typically only seen with DSL customers) but I've talked to people who've got it setup. Outside the US it's a lot more common because of IPv4 address space exhaustion (When a PPPoE connection goes down it immediately loses it's IP address, no waiting for the lease to expire) and because some... authoritarian... governments have had their systems setup for PPPoE inspection since ye olden times.


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PostPosted: Thu May 30, 2019 8:43 am 
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Thanks, guys. Here's an update:

A power outage last night confirms, in my case, it APPEARS that my router retains my ip address. Having suspected that to be the case, I had plugged all of my AC-powered workstation gear into my APC Back-UPS, EXCEPT for my landline phone and its headset, an LED desk lamp and my cable modem. These are plugged via power strip directly into the wall socket. The Back-UPS's alarm woke me during the night, and I found that everything plugged into it was still powered up (the Back-UPS battery was down to about 60%), but the modem was dark. It so happened that, as I was attempting to log into the computer to do a shutdown, the power came back online. So I put the computer – and myself – back to sleep. This morning, everything was normal and I confirmed that my ip address has not changed.

Thanks again for the feedback and tech info!

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PostPosted: Thu May 30, 2019 9:11 am 
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Thanks for filling us in Mike! :)


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PostPosted: Thu May 30, 2019 8:59 pm 
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Your router does not maintain nor store your address. What does so is the DHCP lease to the cable modem from the ISP's CMTS. The CMTS assigns the IP via DHCP leasing. If you maintain power to the cable modem, you can maintain the address. If you have a power outage, one of two things will happen:

1) It's short and you got lucky and the connection came back up before the lease was set to expire (typical leases are 24-36 hours per DHCP lease request and are maintained as much as possible by the CMTS).

2) The lease expired while the power was out and your IP changed. Suckage city, but it happens.

If your equipment is properly grounded/electrically bonded at the demarc (where it enters your house usually), putting the cable modem and router on a UPS should keep your IP steady as they'll maintain power even if the power goes out. But the cable modem is the critical link. It is what the CMTS looks for when the DHCP lease is up. If it can't be found because it's offline, you could well lose your IP. Your router is the LAN facing side that shows you the WAN IP, but the WAN IP is generated by the CMTS and sent through the cable modem. It isn't stored on the router, though a DHCP release/renew request can be sent from the router. The actual IP is stored at the CMTS plant where it's doled out from.

Bottom line: You want that cable modem on the UPS along with the router. And if your phone is of the cordless variety without any form of built in battery backup, you want that too. If your phone service is through the ISP, you cant the EMTA device (they call them telephony devices) on that UPS also. If you lose internet, you lose phone from an ISP supplied line. So the three components you must keep on the UPS are the cable modem, router, and EMTA if you have one. If you're using an ISP's combo gateway (router/cable modem/EMTA all in one), that goes on the UPS.

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PostPosted: Fri May 31, 2019 9:21 am 
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While I have no need for a static, persistent IP address, I do remember the day when you COULD get a kind of guarantee from your ISP of one... but for a fee.


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PostPosted: Fri May 31, 2019 4:15 pm 
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paulc wrote:
While I have no need for a static, persistent IP address, I do remember the day when you COULD get a kind of guarantee from your ISP of one... but for a fee.


That option is now generally only available to business customers, not residential ones. You need a business account, at least on Comcast, to be granted a static IP, and you must use their own supplied equipment for security reasons. They won't supply a static IP to customer owned equipment (CPE).

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PostPosted: Fri May 31, 2019 4:49 pm 
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My first foray was in the early 90s... my company had JUST put in an Internet line (something like half a T1, way faster than my 9600 at home!), I took an old Mac and out up a server, nominally meat for folks within the company. I wanted remote access from my home and the only way the IT guys would let me in was I had a static IP... remember, this is all VERY early stuff, there were only a dozen of so new servers new each week, so I set up something very simplistic for interesting new sites, one for the college editors (John Wiley & Sons) like links to all the schools sites. Word got out and before you know it, my server was getting regularly hit on from a lot of edu's. Corporate got wind of it and some idiot marketing type got "tasked." I built the very first version, all hand coded html, there WERE no tools at all just then. Soon,it got jobbed out to "consultants" for many, many thousands of dollars, but that's a story for many drinks down the road.


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PostPosted: Fri May 31, 2019 7:43 pm 
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With Comcast you can convert to be a Comcast Business customer for about $20/month more (at least in the midwest). Static IPs are available for a monthly fee. The real benefit is there's no bandwidth cap for business customers.


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PostPosted: Mon Jun 03, 2019 11:57 am 
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I have Comcast Business and it's very expensive, although my dynamic IP address has only changed twice in seven years.

- Anonymous


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PostPosted: Mon Jun 03, 2019 3:39 pm 
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Well, it's Comcast. It's expensive, period, unless you're on a promo rate. And they usually make you live with the non-promo rate for a year or two after the promo rate expires, at least by the terms of their normal contract.


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PostPosted: Mon Jun 03, 2019 7:34 pm 
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Anonymous wrote:
I have Comcast Business and it's very expensive, although my dynamic IP address has only changed twice in seven years.

- Anonymous


You can get a static IP, but you're tied to their equipment to do so. They won't do it for CPE (customer purchased equipment). Frankly, it isn't worth them being able to snoop on my LAN traffic to have a static IP.

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PostPosted: Tue Jun 04, 2019 8:12 am 
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Yeah, I considered a static IP when I got the service, but eventually decided I didn't need it, especially at the price they were going to charge. I'm not concerned about their equipment or almost anyone else's since I have Ubuntu Server running on a small Intel box between the cable modem and my network. I do, however, in some small way appreciate that my cable modem doesn't have wireless, so they can't run one of their damned Comcast hot spots off it.

- Anonymous


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PostPosted: Wed Jun 05, 2019 2:15 am 
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Anonymous wrote:
Yeah, I considered a static IP when I got the service, but eventually decided I didn't need it, especially at the price they were going to charge. I'm not concerned about their equipment or almost anyone else's since I have Ubuntu Server running on a small Intel box between the cable modem and my network. I do, however, in some small way appreciate that my cable modem doesn't have wireless, so they can't run one of their damned Comcast hot spots off it.

- Anonymous


The equipment they provide is a combo gateway, meaning it's cable modem and router, and even putting it into bridge mode still lets them snoop packets.

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PostPosted: Wed Jun 05, 2019 10:38 am 
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Squishy Tia wrote:
The equipment they provide is a combo gateway, meaning it's cable modem and router, and even putting it into bridge mode still lets them snoop packets.

Of course, but they can do that with anyone's modem/router because they control the WAN side. They aren't, however, on my LAN because I have my own router between them and my network. Only traffic destined for the publicly routed Internet leaves my router, and they have to be able to see that because it's going over their network. If I had only their device*, then yes, they could see data on my local network.

We live in a world where we must assume all networks are evil. If there's anything I can't allow them to snoop it shall be encrypted, but they definitely can't see any data on my own LAN that isn't destined for the wide open Internet in general.

- Anonymous

* And if it were in bridge mode my LAN would be their WAN, which is a spectacularly bad idea for many reasons. They're doubtless smarter about it today, but in the early 2000s a smallish ISP from which had DSL basically DDOSed its own network due to many customers with bridge mode DSL modems sending LAN broadcast packets across the WAN. It was a very bad network topology, and the bridged DSL modems were very, very dedicated to their bridge mode, to the degree that they didn't filter LAN broadcasts. We had a router plugged in to the bridged DSL, but that didn't help when everyone else didn't. Then ten years later we had a tunnel to bridge two local area networks, and tunnel hardware refused to pass broadcast packets even though we really wanted that capability and it wouldn't have seized up the connection. Sigh.


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