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Russell H.
Russell H., Internet and LAN
Category: Networking
Satisfied Customers: 11066
Experience:  11 years work with Internet/IP, routers, networks, servers
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Customer: ***** ***** - i'm a bit surprised to have someone volunteering help on Christmas!
JA: Thanks. Can you give me any more details about your issue?
Customer: Comcast customer - have been using their modem (Ubee Docsis 3) with Apple Airport Extreme current generation. Sometimes get double NAT error. I'm supposed to set Airport to Bridge Mode, but it never works when I do. Trying to figure out how to proceed. Trying with a brand new Airport and same problem when auto-config'ed.
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Customer: I think I'll keep digging for the moment - Google is your friend. Numerous posts about this, but inconclusive results. May be back - good to know you're here!
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Submitted: 1 year ago.
Category: Networking
Expert:  Russell H. replied 1 year ago.

Hi, thank you for contacting My name is Russell. I will do my best to provide the right answer to your question.

I know this may sound too-simple, as a solution: but try setting the NAT feature on your Airport to Off, period.

That should take care of the 'double NAT' problem.

What are you trying to do with the Airport, exactly? set it up as a router, with WiFi, or something?

Customer: replied 1 year ago.
I'm trying to have my Airport provide dynamic internet address assignment and network service to local devices. The Airport is connected to a Comcast modem.My Airport configuration software v 6.3.6 provides me with router options of "DHCP and NAT", "DHCP Only", and "Off (Bridge Mode)".When I've tried "DHCP Only" before, the system warned me that I may be causing a conflict with my ISP.
Customer: replied 1 year ago.
I've never tried to use "DHCP Only". When I try "Off (Bridge Mode)", I get errors about no DNS and something else that momentarily escapes me. When I use "DHCP and NAT", I get the occasional "Double NAT" error.
Expert:  Russell H. replied 1 year ago.

Try DHCP Only. Your ISP may not know enough about your local configuration or your aims, etc.

(A question: does the modem provide WiFi? if so, that would explain why the ISP warns you against DHCP Only mode.)

Customer: replied 1 year ago.
No, modem does NOT provide WiFi. If I make a network change and this session gets interrupted, how do I re-establish communication with you?
Expert:  Russell H. replied 1 year ago.

Just return to JustAnswer, to this page devoted to your question:

Let me know how it goes, at your convenience. And if we don't get back in touch, Merry Christmas!

Customer: replied 1 year ago.
Attempting to use DHCP-only requires that I reconfigure the WAN IP address."Sharing a range of IP addresses using DHCP Only requires manually configuring your WAN IP address. If your ISP gives you a range of static IP addresses, you should manually configure your base station with the first IP address and distribute the rest via DHCP."That's what I'm anxious about doing.
Expert:  Russell H. replied 1 year ago.

That sounds extraordinary. I never heard of such a thing. Perhaps that is when the Airport is used as the modem as well as the router!

If I'm wrong, well, you may have to talk to your ISP about getting it corrected... or it may be as simple as 'set it to DHCP only, find out that internet is now Down, then set it to DHCP And NAT again, and get it working again'.

Are you willing to take the risk? I don't think it's much of a risk. I never heard of enabling DHCP requiring any such thing. Not with a router off a modem box, where the modem box is the only appliance around that actually 'sees' the ISP-assigned WAN IP Address.

Expert:  Russell H. replied 1 year ago.

BTW, what error condition do you get when you simply set to Bridge Mode ? I suspect some minor setting needs correcting for Bridge Mode to work.

Customer: replied 1 year ago.
When I go to Bridge Mode, I get an error about No DNS. Something else, too, but I don't recall. I may try to recreate them.At the moment, I'm connecting directly using Ethernet from my Mac to the Comcast modem. No WiFi at the moment in the equation. Interesting that I get a different IP address on my Mac when I connect in this manner compared to the address the Airport gets when I connect the modem directly to the Airport.
Expert:  Russell H. replied 1 year ago.

What the other message is, may be quite illustrative of the problem.

You don't have any problem with DNS, or bringing up web pages, in any other mode with this Airport, do you?

Try DHCP mode. That may also straighten out the 'different IP address when connecting directly as opposed to when the Airport connects directly to modem with a cable' issue.

Customer: replied 1 year ago.
Right now, I'm connected via Ethernet to the cable modem and I have this address: Obtained via DHCP. However, when I attach the Airport via Ethernet to the cable modem, I get an address of on the Airport. Also obtained via DHCP.
Expert:  Russell H. replied 1 year ago. on the Airport, is OK. It's in Comcast's range of addresses (for Nashville region.) is also Nashville region's Comcast address range. So you're OK both ways. It's just differently in Comcast's range, not a penalty or a problem.