The server status Unavailable when changing directory server within AD DS

The server status unavailable when you change directory server in AD DS, You should check what if the IPV6 components disabled in Local area connections properties of NIC.

If you want to disable IPv6 components by editing the registry, follow the below steps.

The DisabledComponents registry value affects all network interfaces on the host.

Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, backup the registry for restoration in case problems occur.

To disable certain IPv6 components, follow these steps:
1. Click Start, type regedit in the Start Search box, and then click regedit.exe in the Programs list.
 2. In the User Account Control dialog box, click Continue.
 3. In Registry Editor, locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters\
 4. Double-click DisabledComponents to change the DisabledComponents entry.
 5. Note If the DisabledComponents entry is unavailable, you must create it. To do this, follow these steps:
6. In the Edit menu, point to New, and then click DWORD (32-bit) Value.
7. Type DisabledComponents, and then press Enter.
8. Double-click DisabledComponents.
9. Type any of the following values in the Value data field to configure the IPv6 protocol to the intended state, and then click OK 

10.Type 0 to re-enable all IPv6 components (Windows default setting)
11.Type 0xff to disable all IPv6 components except the IPv6 loopback interface. This value also configures Windows to prefer using IPv4 over IPv6 by changing entries in the prefix policy table. For more information
12. Type 0x20 to prefer IPv4 over IPv6 by changing entries in the prefix policy table.
13. Type 0x10 to disable IPv6 on all non tunnel interfaces (both LAN and Point-to-Point Protocol [PPP] interfaces).
14. Type 0x01 to disable IPv6 on all tunnel interfaces. These include Intra-Site Automatic Tunnel Addressing Protocol (ISATAP), 6to4, and Teredo.
15. Type 0x11 to disable all IPv6 interfaces except for the IPv6 loopback interface.

Disable IPv6 on a specific network adapter You can do this by unbinding the adapter in the Local Area Connection Properties dialog box:

1. 1 Click Start, and then click Control Panel.
2. 2 Click Network and Sharing Center.
3. 3 In the View your active networks area, click Local Area Connection, and then click Properties.
4. 4 On the Networking tab, clear the Internet Protocol Version 6 (TCP/IPv6) check box, and then click OK.

Now you can see in, Domain and Trust  Win2008 DC Status is Online when changing directory server .