The Ipconfig Command Basics

The ipconfig command gets its name from the acronym IP (Internet Protocol) and a shortened term for configure. Understanding this served as a mnemonic device for me when I was first learning networking. It is used to display and configure your connection to the Internet. This tutorial will focus on the display and what that means.

Deciphering the Information

To get to ipconfig, we have to get to the command line.

• Click Start, click Run, type in “cmd” & hit enter.
• Type in ipconfig & hit enter. (you can use ipconfig /all for detailed information)

You will get a screen that looks like this.

To find your router’s IP number, look next to “Default Gateway” (listed last). For a mnemonic, think of the router as your gateway to the Internet or another network. To find your computers’ IP number, look next to “IP Address”. Do not worry about the other information like the subnet mask.

In this case, the computer number is 192.168.1.4 while 192.168.1.1 is the router number. It is common for 192.168.1.1 or 192.168.0.1 to be the router’s IP address. There are a few reasons for this, which were covered in a previous article.

Ping Your Router (Default Gate)

We are using the ipconfig command so that we can get the IP number of the router. As discussed at the end of the last article on the ping command, we want to ping our router. If we can ping it, then it should be working properly.

To do this, type in ping and the router (default gateway) number as shown.

Some Possible Problems

Consider a case where the connection to the Internet seems down. If you cannot ping the router, or the ipconfig command says the default gateway is 0.0.0.0, then the problem may be in the router. The first thing to check is whether the router is on and everything is plugged in. The next thing to try would be to either restart the computer or to try and ping the router from another computer. From here, you may want to call your ISP’s help line and let them know what you have tried. An experienced user may restart the router; I have found that mine needs this from time to time. For a beginner, it is better to get some help. You will learn from the experience.

There is a possibility that your router may be failing. That has happened to me once before. Fortunately, a friend helped me troubleshoot the device, so stay in touch with networking pals. The culprit may likely have been heat from a lack of proper ventilation. Be sure your router has proper air flow. While they may run warm, if they are hot to the touch, it may be bad for the device (check your manual or consult your manufacturer).

ipconfig Command Line Options

Ipconfig has several command line options that you can utilize. You can display all of them with the command ipconfig /?. Here is a short selection of the most common uses.

• ipconfig /release – Releases all IPv4 addresses (requires DHCP)
• ipconfig /renew – Renews all IPv4 addresses (requires DHCP)
• ipconfig /flushdns – Flushes the DNS cache

Linux & Mac Use Ifconfig & Route -n

For those running Linux or are on a Mac, the commands to use are ifconfig and route -n. Is ifconfig identical to ipconfig? No. The ifconfig command will show you your computer’s IP number and other information. You just type in the command, ifconfig, into the terminal to find out information about how your computer connects to the Internet. To get your router’s IP number, you use the route command with a -n (netstat -r should work too). Both are shown in the screen shots. Make sure you use lowercase letter; the commands are case sensitive. (Side Note: Linux also has a iwconfig command for wireless specific tweaking (e.g. frequency).)

Just a note about the Mac ifconfig command. We are using it to view network settings, which is fine. In doing research for this article, I read that it is not advised to use this to change settings. From what I read, it works, but with possible complications.

Closing

We use ipconfig to find the router’s IP number. Once you have that, you can ping the router to test if it is responsive. The problem with the ping command is that, while it is fast, is doesn’t give you a lot of information. For that, we use the tracert command, which will be covered next.

The DNS provider is queried during each connection attempt unless the information are already stored in the cache.

You may want to know why someone would want to flush the DNS cache? Lets say you have decided to switch from your provider’s DNS server to Open DNS, Google DNS or another free service that promise faster speeds and little extras like user-configured site filtering. If you switch during a session, you will be using old data from the cache if you access sites that you have already visited.

Not a big problem? You may be right, but think about the following situation. A webmaster migrates the website to a new server. It can take up to 48 hours before all DNS servers redirect users to the website on the new server and not the old server. Some webmasters change their DNS server before migrating if the ISP’s DNS server is known to be on the slow site when it comes to updating information (another option for webmasters is to edit the hosts file).

The DNS cache can have privacy implications as well, especially on public computer systems or multi-user systems. It is possible to display the contents of the cache, which in turn reveals all the websites that have been visited by the user in the session. Flushing the cache removes the entries so that the next user won’t be able to see a list of visited websites.

Flushing the DNS Cache

On to the commands. Windows users need to open a command prompt to flush the DNS cache. The easiest way to do that is to press Windows-r, type cmd, and hit the enter key.

The command ipconfig /flushdns clears the DNS cache. All records up to this point in time are removed from the cache.

C:\Users\Martin>ipconfig /flushdns
Windows IP Configuration
Successfully flushed the DNS Resolver Cache.

Displaying the DNS Cache

You can test if the DNS cache has been flushed with the command ipconfig /displaydns. Please note that the command may still display entries, but those are new entries that have been added to the cache after it has been flushed. Antivirus software may for instance connect automatically to update servers.

C:\Users\Martin>ipconfig /displaydns
Windows IP Configuration

Turning Off DNS Caching

DNS caching can be turned off for a session or completely. The negative effects are that look-ups may take longer, especially in computer networks with connected network shares and devices. Computer workstations without network connections on the other hand are likely not negatively effected by the operation. It is probably best to experiment with the setting first, by disabling it temporarily for a session.

The command to stop the DNS Cache for the active session is net stop dnscache. Please note that you need administrative privileges to run the command successfully. Windows 7 users can do that by clicking on the start menu orb, selecting All Programs > Accessories, right-clicking on Command Prompt and selecting Run as Administrator. The path should be similar for other versions of Windows.

c:\windows\system32>net stop dnscache
The DNS Client service is stopping
The DNS Client service was stopped successfully.

You can start the DNS client service again with the command net start dnscache.

The previous instructions turned off the DNS cache for a single session. A restart would load it as a service again. It is however possible to turn off the DNS cache completely. This is done with the shortcut Windows-r, typing services.msc and hitting enter.

Locate the service DNS Client in the listing and double-click it. The window that opens offers controls to stop the service for the current session, and to change its startup type from manual to disabled.

Disabled means that it won’t be started with Windows, which effectively disables the DNS Cache permanently. Another benefit of this is that other users are not able to display the DNS cache to retrieve a list of visited websites and servers.

Windows users who are thinking about disabling the DNS Cache permanently should test it first by disabling the cache for the session. If they do not notice performance drops or slower than usual connections they could go ahead and disable the cache completely.

Users naturally feel safer using that mode, but that should not be the case. Why? Because there are means to find out which sites have been accessed in private browsing mode.

You see, one feature of the Windows operating system is a DNS cache, that stores domain name and IP links. Without going into to much details, the DNS cache records information about every website that the user opens in a web browser in Windows.

Curious Windows users just need to list the contents of the DNS cache to find out what websites a user has been visiting in private browsing mode. It may require some additional comparisons to find the private browsing mode websites, but that requires just some manual work and can be neglected.

Here is how you can display the contents of the DNS cache:

• Open a command prompt in Windows. The easiest way to do that is to press Windows-R, type cmd and the enter key.
• Now type the command ipconfig /displaydns in the command prompt and hit enter.
• This displays all websites that have been stored in the DNS cache. Please note that this includes everything, which means websites in all web browsers, regardless of whether they have been opened automatically (e.g. by a script on the site) or manually by the user.

Chance is the list is to large for the command line cache. You can use the command ipconfig /displaydns > dns.txt to save the output in the text document dns.txt. It is then possible to open the document in a text editor, to see all records. Opening it in a text editor has other advantages, like being able to search through the records.

Windows offers an option to flush the DNS cache so that all records are deleted from the cache. This is done with the command ipconfig /flushdns.

Some programs (like CCleaner) offer options to delete the DNS Cache. It is also possible to write a simple batch file to delete it on shutdown. Let me know if you like an example script that does that.

Windows users who regularly work in private browsing mode should consider clearing their system’s DNS cache frequently to protect their privacy.

Users who try to flush the dns cache might receive the error message “Windows IP Configuration Could not flush the DNS Resolver Cache: Function failed during execution” if the DNS Client service is deactivated.

The solution to fix this error message is to enable the DNS Client service again in the Services configuration menu. I’m not sure if it makes that much sense to flush the cache if the DNS Client is disabled as it should mean that Windows is not resolving and storing DNS at all. Experts who might to weight in are encouraged to do so.

You can start the service by typing [Windows R], [services.msc] [enter]. Locate the DNS Client service, right-click it and select properties from the menu.

You need to set the service to manual or automatic before you are able to start the service.