Forums | MacLife

davebot 0.9 · 2009-05-19 6:29 am

I've been enjoying my new Apple AirPort Extreme base station for a couple weeks, and it just works. Then tonight, out of the blue, I had two strange problems. The first was a message that the AirPort had no DNS servers, even though that very pane listed the two numbers for OpenDNS. A few minutes later, this resolved itself, followed by another hiccup a few minutes after that. I'm inclined to ignore this.

The other problem confuses me. I have the AirPort connected to my Comcast cable modem, and it's serving my Mac Pro computer. It serves other devices also, but the Mac Pro is the only one connected right now. I got a 'double NAT' warning, and the AirPort utility says I should change my AirPort from using DHCP and NAT to bridge mode. That makes no sense to me. Even though I have WPA, I'm now wondering if someone else is poaching my connection; the diagnostic was the following:

"Problem 2 of 2: Double NAT

This Apple wireless device has a private IP address on its Ethernet WAN port. It is connected to a device or network that is using Network Address Translation (NAT) to provide private IP addresses. You should change your Apple wireless device from using DHCP and NAT to bridge mode.

- (unselected) Bridge Mode - select this option if your AirPort Extreme is connected to a router or other network device that is already using DHCP and NAT to provide IP addresses to computers on your network.

o (selected) Share a single IP address using DHCP and NAT - Select this option if your AirPort Extreme is connected directly to the Internet with a DSL or cable modem, or to a network with an Internet connection, and you want to use DHCP and NAT to provide IP addresses to computers on your network."

I don't understand this, since my computer *should* be the only device connected, and I need the AirPort to allow my computers and iPhone to connect through my single Comcast line.

Thanks for your help!

davebot 0.9 · 2009-05-19 6:54 am

Just double-checking. . . I grabbed my iPhone, which was sleeping, and I looked up its MAC address. Opening the AirPort Utility again, I found the only two devices connected were my Mac Pro and the iPhone. Since I told the AirPort to ignore the previous problems, it's no longer complaining. It bugs me that I don't know why it was complaining about a double NAT problem. (It's just the sort of thing that seems to happen to me when I wake up in the middle of the night and use my computer!) Is there anything more I should do?

Thanks in advance. . .

dv · 2009-05-19 8:40 am

Double NAT happens when plugged into a cable or dsl modem that also is doing NAT. This is pretty common.

Putting the Airport into "bridge" mode will resolve the issue.

Nobody is poaching your connection.

If you plug the Mac pro directly into the Airport, does it pull an external (72.x.x.x, 98.x.x.x, or something) IP, or does it get a local/LAN IP from the modem? (those typically start 10.x.x.x or 192.168.x.x)

davebot 0.9 · 2009-05-19 11:10 am

Just checking now, I find that my iMac reports an IP Address of 10.0.1.4 and the Mac Pro reports 10.0.1.3, with the Router at 10.0.1.1.

My concern was that I've been told that Comcast tells customers to purchase extra IP addresses if they have multiple devices using the same IP. But everything's supposed to be fine if you have a router taking the single IP they give you and serving up addresses for the devices on your home network. I wasn't aware of any such activity from the cable modem (a new Motorola Surfboard SB5101).

Ok, it turns out that my cable *is* long enough. I just tried connecting it from the AirPort to the outer ethernet socket of the Mac Pro -- and nothing at all happens! Oh, oh. Is it broken? So I tried putting it in the other socket, and I see that I'm connected, with IP Address 10.0.1.5. Now that I look back there with a flashlight, the socket that works is labeled "1" and the one that didn't work is labeled "2." (I thought both worked before. . .).

Sooo. . . what's this mean, kind Sir?

Thanks,

Dave

dv · 2009-05-19 11:14 am

Sorry, I misspoke - plug the mac pro into the cable modem, not the airport.

Unless your Airport base station is defective, all the ports on the back should work.

davebot 0.9 · 2009-05-19 11:15 am

When I plug the ethernet cable into socket 2, System Preferences says "Cable Unplugged: The cable for Built-in Ethernet is not plugged in." What?

dv · 2009-05-19 1:15 pm

Assuming you're plugging the computer into one of the three LAN ports "for computers and devices" and not the WAN port, I'll say it again:

Unless your Airport base station is defective, all the ports on the back should work.

Stuff happens, so if it's under warranty, no big deal. However, intermittent or sketchy operation is a pretty good indicator of impending total failure.

Do a factory reset of the router and see if that helps.

dv · 2009-05-19 1:19 pm

But seriously, what does your mac pro pull as an IP when you plug it into your modem? (leaving the base station out of the scenario entirely.)

davebot 0.9 · 2009-05-19 9:26 pm

Thanks so much for your help! First things first. I didn't mean that I plugged the cable into port 2 of the *AirPort*. I meant that when I plugged it into port 2 of *the Mac Pro* nothing happened. I'm worried that I may have a bad Ethernet card in my Mac Pro. . .

As for the other question. . . let me now turn off wireless and connect the cable directly to the cable modem. . . Okay. I connected the cable modem directly to the Mac Pro's 1st Ethernet port. Pulling up the Network preferences, I see "Built-in Ethernet" and "Self-Assigned IP" and "Status: Connected Built-in Ethernet has a self-assigned IP address and may not be able to connect." It has IP Address 169.254.58.43 and Subnet Mask 255.255.0.0.

My Internet browser reports "Cannot Load Address no Internet connection."

Then again, Comcast has this dependence on the Mac address of the device connected to the cable modem. Since the Mac Pro's ID is different than that of the AirPort basestation, I would have predicted that this wouldn't work. It may work if I do what I did before: turn the cable modem off for 20 minutes, and then connect and power everything step-by-step. For now, I think I'll re-connect the AirPort so I can send this.

Thanks,

Dave

dv · 2009-05-19 11:49 pm

Ah, I see.

In a perfect universe, you could clone the MAC of the computer on the Airport, give Comcast your computer's MAC, and you'd have two different devices that would work with your cable modem - just in case you need to troubleshoot. Unfortunately, the APEBS is one of the only routers on the market that doesn't have that feature.

It's still pretty likely that you're getting the NAT error because something upstream is also doing NAT, just like the error message says. It's probably your cable modem. As you've noticed, it's not a huge problem, but it's not recommended.

What happens when you set the airport to bridge mode and connect the computers to it? Do multiple devices still operate correctly?

davebot 0.9 · 2009-05-20 1:07 am

Well, I did the experiment! In AirPort Utility, I went to the Internet section, selected Internet Connection, and set Connection Sharing to Off (Bridge mode). The Mac Pro and the iMac then claimed that they were not connected to the Internet. The iPhone still connected, slowly, as a phone. Setting Connection Sharing back to "Share a public IP address" restored functionality.

I also looked at the CDROM info for the Motorola cable modem. I found a page talking about connecting multiple computers, but the way to do that required an Ethernet crossover cable to an Ethernet hub or switch that multiple computers connect to. So it doesn't look like it's doing NAT. It's different from my old DSL modem, which I could access through a Web browser and play with settings. I don't know how to do that with this guy.

It also seems mysterious that AirPort Utility also complained briefly that it had no DNS servers.

By the way, my 2008 Mac Pro has two Ethernet sockets on the back. Connecting a cable to #1 worked, and connecting to #2 didn't. But looking at the system profile info, I don't think I see two Ethernets. I certainly didn't order anything special in that regard. So do you think this is normal behavior?

Thanks for all your help, by the way!

Dave

dv · 2009-05-20 9:14 am

davebot 0.9 wrote:
I also looked at the CDROM info for the Motorola cable modem. I found a page talking about connecting multiple computers, but the way to do that required an Ethernet crossover cable to an Ethernet hub or switch that multiple computers connect to. So it doesn't look like it's doing NAT.

What is the airport base station's external IP address? (Internet tab of Airport Utility.)

If you can connect multiple computers by using a crossover cable and a hub or switch (as opposed to a router) then yes, the modem is assigning IP addresses.

However, if Comcast has only assigned you a single IP/MAC combination, then you'll have to have the Airport there and the double-NAT issue will remain unresolved.

I don't know about the Mac Pro ethernet thing, never played with one.

davebot 0.9 · 2009-05-20 11:03 am

Thanks!

Here's the Internet Conenction info from AirPort Utility:

IP Address: 98.237.204.55
Subnet Mask: 255.255.248.0
Router Address: 98.237.200.1

It's true that Comcast gives me a single IP / MAC combination. I thought I might find a setting in the Motorola cable modem to turn off NAT, but I haven't found anything like that in the manual, and I haven't discovered how to connect to it to play with settings. So it's more of an 'appliance' than my DSL modem was. I'm enjoying the fast connection, though.

Thanks again,

Dave

dv · 2009-05-20 12:59 pm

Judging by that info, you can't connect to the modem to change settings. Whatever NAT your airport is complaining about is happening outside your control - and probably outside your house. So it's not something you can worry about, I guess.

davebot 0.9 · 2009-05-23 3:36 pm

Thanks DV! Given that this is Comcast, I'm not really surprised. But they're treating me well and my network is fast, and my router now seems to be happy, so I'm happy too. Thanks for sharing your expertise.

--Dave

Forums | MacLife

#1 2009-05-19 6:29 am

Airport Extreme "Double NAT" problem -- huh?

#2 2009-05-19 6:54 am

Re: Airport Extreme "Double NAT" problem -- huh?

#3 2009-05-19 8:40 am

Re: Airport Extreme "Double NAT" problem -- huh?

#4 2009-05-19 11:10 am

Re: Airport Extreme "Double NAT" problem -- huh?

#5 2009-05-19 11:14 am

Re: Airport Extreme "Double NAT" problem -- huh?

#6 2009-05-19 11:15 am

Re: Airport Extreme "Double NAT" problem -- huh?

#7 2009-05-19 1:15 pm

Re: Airport Extreme "Double NAT" problem -- huh?

#8 2009-05-19 1:19 pm

Re: Airport Extreme "Double NAT" problem -- huh?

#9 2009-05-19 9:26 pm

Re: Airport Extreme "Double NAT" problem -- huh?

#10 2009-05-19 11:49 pm

Re: Airport Extreme "Double NAT" problem -- huh?

#11 2009-05-20 1:07 am

Re: Airport Extreme "Double NAT" problem -- huh?

#12 2009-05-20 9:14 am

Re: Airport Extreme "Double NAT" problem -- huh?

davebot 0.9 wrote:

#13 2009-05-20 11:03 am

Re: Airport Extreme "Double NAT" problem -- huh?

#14 2009-05-20 12:59 pm

Re: Airport Extreme "Double NAT" problem -- huh?

#15 2009-05-23 3:36 pm

Re: Airport Extreme "Double NAT" problem -- huh?

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