Private IP addresses Cablevision


I recently switched to optimum online from DSL. My Linksys router was configured with a static private IP address on the public interface (192.168.x.x). After connecting the router to the cable modem, I was able to browse without any problem. I then changed the configuration on the router from static to DHCP client and got a public IP address. What I dont understand is how did I connect the Internet with a private IP address?

I would appreciate if anyone can help me solve this mystry.

Thanks in advance


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The router Device IP Address on the Setup page is only applicable to the LAN side. Your DSL ISP issued a WAN IP address that was not in the private 192.168.X.Y range. A private address on a public network is meaningless (by definition) since many might choose the same IP address as you, bringing the Internet to its knees. You can set the routers LAN address to whatever you choose, the WAN doesn't use it and doesn't care what it is. On the WAN side many cable companies register only the MAC address of the router and assign an IP address to it on their system, there is no ability to specify the WAN IP address from the router in gateway mode (connected to the Internet). Even if you contract a business-class static IP address, the static IP address is assigned by the ISP, neither you nor the router has any control over it. The DHCP toggle in the router's DHCP page is its own server to the LAN only. If you were running only static LAN IP addresses, you could turn this off.

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khel (myname) wrote in news:9194ddcf.0409081827.488c9ed6

Your ISP may have had NAT running as well. So in esscence your connection looked like this:

Router -> Nat -> ISPs Router -> NAT -> ISPs Internet Router -> Internet

So basically NAT to NAT to Public.

Reply to
Lucas Tam

Unless they've changed it in later firmware from what I have in v1.39, you can specify both the LAN side and the WAN side IP on the Setup page.

If he did in fact specify a WAN IP address, this would not happen.

Well, not meaningless. A private IP address just isn't routable. It only would have an adverse effect if another device downstream from the first router encountered was using that address. Devices with non-routable IP addresses will not be able to communicate beyond that router.

But he says he's not talking about the LAN IP. He said he set a static IP on the WAN side.

Irrelevant to this situation. Provisioning is not the issue.

Yes. You can specify the WAN IP address.

If the ISP assigned him a static IP, he would manually enter it on the Setup page of the router. He wouldn't have control over what the ISP tells him it will be, but he does have control over typing it in correctly.

That is true. The DHCP tab controls only the LAN IP address.

To turn off DHCP on the WAN side, one toggles from "Obtain an IP Address Automatically" to "Specify an IP Address" on the setup page.

I suspect that what happened was the OP was actually looking only at the DHCP tab, which only controls the LAN side, and not at the Setup tab. When you specify a WAN IP address on the Setup tab, you also need to specify the subnet mask, the default gateway, and at least one DNS server. The OP may have done this, or it may be a significant clue that he didn't mention these settings, leading to the conclusion that he was really playing with the LAN IP address, not the WAN IP address.

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Linksys BEFSR11, Firmware Version: 1.46.00, Jun 24 2004. On my setup page I can set the LAN IP address, and I can choose among several options for the WAN IP address; one of those options is: Static IP. Would you know how to configure that IP address? (I used that option on an SMC Barricade, and set the WAN IP address to an RFC 1918 IP address, once. It worked, too.)

Actually, the Internet would never notice an RFC 1918 IP address was even being used. The first router which handles publicly routed packets would just ignore an IP address for which no routing is announced. (This includes bogons, and some IANA assigned IP addresses whose owners don't announce public routes.)

My Linksys BEFSR11 is currently using on the WAN. It does use this RFC 1918 address, and it does care what it is.

While it is true that some cable companies use MAC registration, that seems to be declining. And it is entirely possible to manually specify an RFC 1918 IP address on the WAN port of most routers; I did it with my SMC Barricade

7004BR. Of course, if your ISP is providing an IP address by DHCP lease, or MAC registration, or DOCSIS certificate registration (is that how the cable companies that don't use MAC do it now?), and you pick your own IP address out of thin air, you won't get onto the Internet.

Of course. If you are buying Internet service from a provider, you must accept their IP address.

There are other arrangements, as well. My LAN uses DHCP assignment from a Netgear FR114P. I can make an alternate dial-up connection over a modem attached to an SMC Barricade 7004BR. The Barricade has DHCP turned off, and is hard set to The Netgear has a MAC reservation for the Barricade on

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I wish I could understand what you described, but it is clear as mud. In general, ISPs issues IP addresses from their IANA assigned public IP address pool. However some ISPs actually use an internal NAT system, assigning RFC 1918 IP addresses to their customers, and connecting those customers to the Internet through an ISP run NAT router.

The part of your description which puzzles me is that you claim to have changed your WAN IP address, and still gotten onto the Internet. As another poster has mentioned, your Internet connection is under the control of your ISP; they control the IP address you are assigned for Internet connection.

Frankly, if your DSL provider was using a static RFC 1918 IP address, and your cable company is using a DHCP public IP address, your rig should not have worked at all until you made the requisite change in your router.

WAN is a confusing term. Strictly speaking, it means "Wide Area Network", and refers to a network of larger scope than a LAN ("Local Area Network"). While the Internet is a WAN within the definition of the term, there can be networks intermediary to the Internet, and your LAN, which can be considered a WAN. You encountered such an arrangement with your DSL provider. I have created such an arrangement within my home; though WAN is usually applied to a network which spans multiple premises.

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Some ISPs also use RFC1918 addresses for internal routing of public addresses.

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James Knott

That concept seems to be a bit fuzzy to me. Perhaps you could offer an example?

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Here's one. In this example, the second hop is my ISP @, between me and their nearest office, which is listed as the 3rd hop at Passing through the RFC1918 addresses does not cause a problem, so long as it's use is with the ISPs internal network, in that there should not be any traffic to those addresses from the customer. The 1st hop, is my linux firewall @

traceroute to

formatting link
(, 30 hops max, 40 byte packets 1 firewall.home ( 0.798 ms 0.494 ms 0.531 ms 2 8.074 ms 10.239 ms 13.006 ms 3 ( 19.090 ms

17.230 ms 32.273 ms 4 ( 7.077 ms 10.930 ms 11.950 ms 5 ( 14.937 ms 14.999 ms 6.112 ms 6 ( 31.996 ms 34.491 ms 39.881 ms 7 * * * 8 ( 41.644 ms 45.267 ms 42.918 ms 9 ( 41.142 ms 41.656 ms 39.813 ms 10 ( 41.631 ms ( 40.042 ms 41.675 ms 11 ( 40.506 ms 43.528 ms 41.652 ms
Reply to
James Knott

Ah, I get it. They aren't exactly "routing public IP addresses through RFC

1918 IP addresses. They are using internal routes between your computer and the Internet. It appears that your 2nd hop router is a bridge; your cable modem, perhaps?
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Possible, but it doesn't matter, so long as those RFC1918 addresses are used interally and not used for directly accessing some service. There is nothing special about those address ranges, other than how the routers are configured to handle them. Because they're supposed to be blocked from the public internet, they can be used in multiple locations. Also, I seem to recall that using such addresses is part of the DOCSIS standard, but not sure though.

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James Knott

Forgot to mention, there's absolutlely no harm in an ISP using those addresses interally for routing any public traffic, so long as they're not being used, where the ISP connects to the internet. For example, my ISP has locations across the country. There's no reason why they couldn't pick up traffic from an ISP in one part of the country and carry it via RFC1918 internal network, to another ISP in a different area.

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