Gigabit Ethernet through AirPort Extreme

If the AirPort Extreme 802.11ac is set up as a DHCP and NAT router, then the WAN to LAN speed is going to be limited to about 300-400 Mbps. Check the disappointing results here:

http://www.smallnetbuilder.com/tools/charts/router/view

If the AirPort Extreme is set up in Bridge Mode, then you might expect to see 600-700 Mbps speeds that way.

http://www.smallnetbuilder.com/tools/charts/router/bar/75-lan-to-wan

If you are not seeing comparable results, then the AirPort Extreme is not negotiating speeds correctly with your modem. Unfortunately, Apple removed all of the settings to manually adjust WAN port speeds on the 802.11ac AirPort Extreme, so there is not much that you can do other try a few unusual things.......

One of them would be to insert a Gigabit Ethernet switch between the modem and AirPort Extreme to see if it will negotiate speeds better that way

Another would be to try a crossover Ethernet cable between the modem and AirPort Extreme.

If none of these things help, then you should ask Apple to replace the AirPort Extreme for you and hope for better luck on the next try.

Although the latest AirPort base stations have 1 Gbps Ethernet ports, they average around 575-600 Mbps of total simultaneous throughput via their WAN/LAN interface (bridge mode) ... so I wouldn't expect that you will see anything like the performance you're seeing just through the modem alone. If getting 800+ Mbps is your goal you may want to consider routers provided by other manufacturers at this point in time.

Are you sure you are using airport extreme and not airport express?

Have you tried either option Bob has provided yet? That is, using a Gigabit Ethernet switch or crossover Ethernet cable?

A "crossover" cable has one of its wire pairs switched at the connector. The more common cable is the "straight-thru" type were, as the term implies, all pairs start and end at the same terminal number on the connector ends.

Bob, I believe, asked you to try this to see if it will help your base station negotiate the best possible connection between itself and your modem. Here's one source for this type of cable.

I'd have to buy a gigabit switch. I might consider that

Most stores will allow you to return a device if it does not do what you want, but it would be a good idea to ask to make sure that the store will allow you to return the switch if it does not help.

but don't understand the distinction between a crossover cable and a regular ethernet cable.

A crossover cable has a different wiring scheme than a regular Ethernet cable. Some time ago, most manufacturers required that you use a crossover cable between the modem and router. That is not really needed anymore, but the slight change might result in the WAN port on the AirPort Extreme being able to negotiate speeds with the modem more effectively.

We've seem some posts from users who have reported that using a crossover cable really helped their performance.....and some have said that it had no effect at all.

Same advice as above.....check with the store to make sure that you can return the cable if it does not help.

Since you are checking speeds between the AirPort and iMac.....I am assuming that you know that the Ethernet cable is working correctly in that location. A bad cable would still allow a connection.....but it would drop a potential 400+ Mbps connection down to less than 100 Mbps.

The bottom line when you are having problems with Ethernet connection speeds........most of the time, the issue is a bad cable somewhere.

Connect via 5 ghz,

range is bad but speed is impressive

On 2.4 ghz i get 40 50 megabit download speed, with 5 ghz i get the full 250 megabits the isp allows for my plan