I had a WRT54G in this exact situation.
- Ensure your modem / router (in your case a Comcast Modem) to hand out DHCP addresses.
- Ensure your modem and your Linksys both have IP addresses on the same subnet, but different addresses (192.168.1.1 and 192.168.1.2 are good examples)
- Disable DHCP on your Linksys.
This is a sort of 'pass-though' - the Linksys will route all DHCP requests through to the rest of the network - where the router will catch them.
Since the rest of the network is on the same LAN as the Linksys, it shouldn't be given any dramas by NAT. Just don't use the WAN port.
I've used this exact setup for several years, and it has worked flawlessly. I can even take my wireless point to a friend's house and use it - especially if he has a different subnet, it won't interfere, and doesn't require any setup (again, the pass-through effect).
Edit: And if you have need of a 5th port, the WRT54G has the ability to use the WAN port as a LAN port. I'm not sure where it is, it's been a while since I replaced my WRT54G, but it's a superb unit.
The message:
There is a "Double NAT" in the network. This AirPort base station has a private IP address on its Ethernet WAN port. It is connected to a device or network that is using Network Address Translation to provide private IP addresses.
is purely a warning message, which you can safely disregard altogether.
The annoying feature of this configuration is that none of the devices on the 192.168.0.0/24 network will be able to talk to those on the 192.168.1.0/24 network. In other words, right now they represent two distinct subnets. You may keep this configuration, if this is of no concern to you.
Alternatively, you may wish to integrate all of your components into a single subnet. You may obtain this in this way (the order of these operations matters):
1.connect to the Airport, and disable DHCP;
2.unplug the ethernet cable from the WAN side of the Airport, and plug it into the LAN side.
3.open the GUI of the main router, and change the mask of the LAN, from 255.255.255.0 to 255.255.254.0.
4.Turn off everything;
5.turn on the router, count to five, turn on all of the devices in whichever order.
By doing this, you will have established a twice-as-large network, with these properties (output of ipcalc):
# ipcalc 192.168.0.0/23
Address: 192.168.0.0 11000000.10101000.0000000 0.00000000
Netmask: 255.255.254.0 = 23 11111111.11111111.1111111 0.00000000
Wildcard: 0.0.1.255 00000000.00000000.0000000 1.11111111
Network: 192.168.0.0/23 11000000.10101000.0000000 0.00000000
HostMin: 192.168.0.1 11000000.10101000.0000000 0.00000001
HostMax: 192.168.1.254 11000000.10101000.0000000 1.11111110
Broadcast: 192.168.1.255 11000000.10101000.0000000 1.11111111
Hosts/Net: 510 Class C, Private Internet
This way you will have defeated the dearth of IP addresses, while still keeping all of your devices into the same subnet. Turning everything off and on again forces the introduction of the new mask, and the new routing table.
This works provided you do not have static IPs. If instead you do, you will have to manually adjust the mask to reflect its new value, on each device.
Best Answer
If you have NAT on, on both devices, then they will both be doing NAT on all outgoing/incoming frames, and thus will both cause a performance hit. It's not like having the AirPort Extreme doing NAT first is somehow taking some of the load off of the DSL modem if the DSL modem is doing NAT as well.
Turning off NAT on your AirPort Extreme will keep your AirPort Extreme from adding a small performance hit to your traffic, but I agree with your sense that leaving your TG508v2 to do NAT is probably a suboptimal solution. I'm guessing the NAT engine in the AirPort Extreme is higher quality than the one in the DSL modem, so if there's any way to get your DSL ISP to switch out your TG508v2 with a bare-bones modem that doesn't do NAT, that's the best way to go.