Author Topic: TF7100HDPVRt and Apple Airport Extreme  (Read 2845 times)

Offline ferret

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TF7100HDPVRt and Apple Airport Extreme
« on: August 29, 2009, 03:16:09 PM »
I have a home wireless network based around an Apple Airport and can't get the Topfield device to connect. It appears to be able to see the network but not connect. Does anyone have experience with this or provide advice? (Any help would be appreciated as my wife is threatening to send it back)

Offline billlee

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Re: TF7100HDPVRt and Apple Airport Extreme
« Reply #1 on: October 18, 2009, 11:20:11 AM »
In my experience this may be related to your possibly using NAT inside a network already using NAT (Network Address Translation).

NAT is a neat idea that gets around the lack of IP4 addresses, as well as enhancing security by not making your home/office devices open to the world (and hacking attacks).

How it works:
Everything on your local (home/office) network is given an IP number that is part of a set of IP numbers reserved for this use. A common range is [192.168.0.x] where x changes for devices on your local network. From outside the network, none of your local devices on your network are visible, since the Internet deliberately doesn't pass data packets with these reserved source or destination IP addresses. Your NAT server has an public IP address that is visible to the world, and a bunch of private IP addresses that it uses on your local network.

When your 7100 is attached to a network, it normally looks for a DHCP/NAT server, so it can get a valid IP address. The DHCP/NAT server knows the IP number where your 7100 is, and so when it makes a request for, say, the TV guide, a data packet requesting that goes to the Gateway IP address, normally the same as the (DHCP server/NAT server/modem) in small networks. The data packet gets sent along to the ICETV server, but replaces source address of the data packet with that of the NAT server.

The ICETV server receives the data packet, and sends back a reply to the address contained in the request, which is that of your NAT server. When your NAT server gets the reply, it knows to pass it on to your 7100, and not some other device that is on your local network because it has kept a record of where that original request came from. It passes on the reply packet, but replaces the destination IP address from the NAT server's externally visible address with the internal local address of your 7100.

This usually works well, with the NAT server taking packets from devices on the local network, replacing the source IP number with that of its own externally visible IP address, and when it gets a reply packet, replacing the destination address of the NAT server with that of the local IP number and sending the packet to the local network for delivery. The security comes in because data packets that are not in response to outgoing requests reach the NAT server and it discards them since it hasn't got a destination IP address for this unsolicited data.

Apple Airport Extreme
If you use an Apple Airport Extreme, you are likely using it as a DHCP/NAT server in its role as a wireless point. It is likely that you also have a Internet modem that is also acting as a DHCP/NAT server. Both would be normally doing service as NAT servers as part of their DHCP service. If you have a 7100 connecting wirelessly through an Airport Extreme which is connected through your ADSL modem to the Internet, then it is likely you are set up behind two NAT servers: the Airport extreme, and your ADSL modem.

Some devices just don't like being behind two NAT servers, possibly being confused especially if they use the same internal IP address space. This setup is known as a 'Double NAT' setup.

The usual fix for this is to have just your Internet modem being the DHCP server, and getting the Apple Airport Extreme modem to pass on DHCP requests to your modem and not handle them itself.

You can determine whether you have such a Double NAT arrangement by checking the Apple Support article at This link tells you also how to configure your Airport Extreme how not to provide NAT services for items connecting through its wireless connection.


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