CompTIA Network + Certification Exam Tutorial:

Troubleshooting Exercise: User Cannot Access Internet

By Chris Bryant, CCIE #12933

Part of passing the Network+ certification exam is knowing how to "spot the issue" - and that's exactly the kind of thing we do on a day-to-day basis! In this new series, you'll have the opportunity to spot basic networking issues that often pop up, which in turn gives you valuable practice for the Network+ exam!

In today's exercise, we'll take a look at a situation where our end user can't get to the Internet. Since we all know how calmly our end users usually take this news, we had better spot this issue quickly! :)

When the end user says "sometimes I have trouble getting to something, sometimes I don't", that doesn't give you much of a starting point.  When I hear that, I always go straight to ipconfig /all.  Let's take a look at the following example -- do you see the problem?

C:\>ipconfig /all

Windows IP Configuration

        Host Name . . . . . . . . . . . . : your-4dacd0ea75
        Primary Dns Suffix  . . . . . . . :
        Node Type . . . . . . . . . . . . : Hybrid
        IP Routing Enabled. . . . . . . . : No
        WINS Proxy Enabled. . . . . . . . : No
        DNS Suffix Search List. . . . . . : hsd1.va.comcast.net.

Ethernet adapter Local Area Connection:

        Connection-specific DNS Suffix  . : hsd1.va.comcast.net.
        Description . . . . . . . . . . . : Intel(R) 82562V 10/100 Network Connection
        Physical Address. . . . . . . . . : 00-17-31-F7-03-B2
        Dhcp Enabled. . . . . . . . . . . : Yes
        Autoconfiguration Enabled . . . . : Yes
        IP Address. . . . . . . . . . . . : 68.57.34.228
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 68.57.43.1
        DHCP Server . . . . . . . . . . . : 68.87.73.12
        DNS Servers . . . . . . . . . . . : 68.87.73.242
                                            68.87.71.226
        Lease Obtained. . . . . . . . . . : Thursday, September 28, 20xx 8:30:29 PM
        Lease Expires . . . . . . . . . . : Monday, October 02, 20xx 2:00:06 PM

The problem here is the default gateway.  The PC itself is on the 68.57.34.0 255.255.255.0 network, so the default gateway's address should be on that same network.  Instead, it's 68.57.43.1, meaning someone just typed a little fast when they were setting the gateway.  It happens.  :)  

Another common configuration error is assigning an incorrect subnet mask.  If you see a Class C address, you should see a Class C mask.  Assigning the wrong mask to addresses (Class A mask for Class B networks, for example) can lead to connectivity issues.

ipconfig /all is really the first command you should run when troubleshooting from a network.  It lists the DHCP and DNS server locations as well as the default gateway - can the PC ping all of those?  If not, there's a problem.  Does the PC have a valid lease?  Just check the dates at the bottom of the output, and if there are no dates and the PC is set to use DHCP, there's really a problem! 

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com