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Unless you are currently using IPv6, be sure that your AirPort Extreme is configured to use the “Link-local only” setting for IPv6.

You can set this using the AirPort Utility:

AirPort Utility > Select the Extreme > Edit
Internet > Internet Options > Configure IPv6: Link-local only

On your Mac:

System Preferences > Network > Wi-Fi > Advanced
TCP/IP > Configure IPv6: Link-local only

Find out what is preventing your Mac from going into sleep mode by running the pmset -g assertions command. In the example below, the PreventUserIdleSystemSleep is set to 1, which means the system is being kept from going into sleep mode. The com.apple.ppp.pptp and com.apple.ppp.l2tp are the process that are preventing the system from sleeping. Disabling the VPN server allowed the system to sleep.

echelon:~ dervish$ pmset -g assertions
10/20/13 8:32:17 AM MST
Assertion status system-wide:
PreventUserIdleDisplaySleep 0
PreventSystemSleep 0
PreventUserIdleSystemSleep 1
InternalPreventDisplaySleep 0
ExternalMedia 1
DisableLowPowerBatteryWarnings 0
UserIsActive 0
ApplePushServiceTask 0
BackgroundTask 0

Listed by owning process:
pid 36(powerd): [0x0000000900000249] 17:13:31 ExternalMedia named: “com.apple.powermanagement.externalmediamounted”
pid 67(com.apple.serve): [0x0000000100000137] 25:56:48 PreventUserIdleSystemSleep named: “com.apple.ppp.pptp”
pid 67(com.apple.serve): [0x0000000100000136] 25:56:48 PreventUserIdleSystemSleep named: “com.apple.ppp.l2tp”
pid 8690(httpd): [0x00000001000006dc] 11:54:12 PreventUserIdleSystemSleep named: “com.apple.apache.denysystemsleep”

Error:
“The iPad [device name] could not be restored. This device isn’t eligible for the requested build.”

1. Update to latest version of iTunes.
2. Remove/comment out gs.apple.com entry from /private/etc/hosts.
#74.208.10.249 gs.apple.com
3. Restore device.

Issue:
When connecting to a new (or newly reset) AirPort Extreme/Time Capsule, the AirPort Utility fails to connect and returns Error -6753.

Solution:
Enable IPV6 on the client machine you are trying to manage the AirPort Extreme from. If you have disabled IPV6 on the client machine, you will not be able to perform the initial configuration of the AirPort Extreme/Time Capsule.