“The RPC server is unavailable” while logging to Domain controller OR joining client machine to Domain Controller
There are many reasons to see this error,
I will discuss the issues that mostly make this happen

1] RPC service might be disabled.
Start the service through “services.msc”

2] RPC Dependencies services might be stopped
Start Dependencies services for RPC

3] You have client installed through OS image restore
Change the SID of Client machine using NewSID v4.10, its and utility provided on Microsoft site

4] Check the firewall between Domain controller and client machine for RPC port 135 must not be blocked
Unblock this RPC port for TCP

5] Check the SRV record for LDAP in DNS
Check for SRV record must be present [probably it is DC IP address]

6] Check for DNS configuration are proper and with out any error
Run “NetDiag /fix” to fix the issue on DNS server
To check the DNS zone for your new domain controller,
In Run type dnsmgmt.msc.
Expand Computer Name, expand Forward Lookup Zones, and then expand Domain.com
Confirm that the _msdcs, _sites, _tcp, and _udp folders exist.
After running “NETDIAG /FIX” command.
The most important test you will see is the first one, which is connectivity. This test will tell you if your domain controller is properly registered in DNS. If your tests are successful, you have a healthy domain controller. If this command shows “FAIL” result for any Diagnosis that is running on server then reboot Domain Controller and Re-Run this command
A typical successful output will start as follows:
Domain Controller Diagnosis

Performing initial setup:
Done gathering initial info.

Doing initial required tests

Testing server: Default-First-Site-NameAD01
Starting test: Connectivity
……………………. Domain.com passed test Connectivity

Tip “NETDIAG /FIX” command is present in 2003 support tools

It is very easy to block a single IP address on a Linux server but Windows Default firewall doesn’t allow us to block a single IP address on the server or for particular ports. However Microsoft released this and introduced IP Security Polices in their Local Security Polices option in the release of their SP2. But most of us are not aware of this option and we mostly adopted third party firewall and pay for their heavy license. However Firewall onl Windows 2008 Server is far more advanced than that on Windows 2003.

It would not be the case now as I have listed detailed steps along with the images on how to block IP using the IP security policy in Windows. This option is also available in XP as well as Windows 2003 Server edition.

How to BLock IP Using Windows:

You can either open MMC from START >> RUN >> MMC and add a new Snapin for IP Security policy with steps below:

Click ‘Start’ > ‘Run’ >type ‘MMC’ press ok.
In the console click > ‘File’ > ‘Add/Remove Snap in’
In the ‘Standalone Tab’ click The ‘add’ button
Seclect ‘IP Security Policy Managment’ > ‘ADD’ > ‘Local Computer’ > ‘finish’ > ‘close’ > ‘ok’
You should now be back to the Management console.

OR

Just goto START >> PROGRAMS >> ADMINISTRATIVE TOOLS >> LOCAL SECURITY POLICIES ON LOCAL COMPUTER to open the IP Security Management Console.

1. Select IP Security Policy and Right Click on the right pane to select new Policy. The screen will like an image below:

Figure 1

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Difference between Hardware firewall and software firewall
Which firewall is best?

Well this question again generate a cross question.
What are your requirement?

If you need to protect your network through IP based regardless of Application using the port. Hardware firewalls has a strong and fast protection from most forms of attack coming from the outside world. Hardware firewall has a designed technique called packet filtering that examines the header of a packet to determine its source and destination addresses [IP’s].  This information is compared to set rules that determine whether the packet is to be forwarded or blocked
Now days a new technique called Stateful Packet Inspection (SPI) is used in firewall
That check the origin of packets that means it check whether the response is from internet, if yes whether it is the requested response for a machine from local network.
If yes the allow traffic
Bad thing is that, if, say, you get email containing WORD doc that has spyware/Macro hidden in it, which communicate to outer world when WORD file is opened/executed.
Your hardware firewall will allow communication to outer world and the requested incoming packets too, as the packet origin was from local network,
Resulting Machine infection

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Creating a VPN Server on Windows will require enabling Routing and Remote Access Service (RRAS). This service will not start unless you have at least two network connections. The purpose of RRAS is to pass packets between IP subnets. It basically turns your Windows workstation or server into a router. Normally, when you have only one network interface, any attempt to enable the service will fail.

To get around this requirement, a virtual network adapter can be installed. This software-based NIC is called Microsoft Loopback Adapter. To install the Loopback Adapter, you can follow the instructions below. This step-by-step, like all the others in this article, follow the wizards and consoles installed in Windows 2000. The wizards and tools on Windows XP Pro and Server 2003 are similar. The instructions below provide enough information to configure utilities on those operating systems with very little modification.

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