We all know to configure routers and three switches must be careful when routing, IP address and subnet mask are not a little mistake. Recently, I discovered during debugging a network IP settings of the machine will result in improper routing anomalies.
Fault Description
I always like to network unit debugging, the company recently launched a Web site requires the external debug access connectivity. I use the laptop in the company's server farm to do a debug and modify the IP address of 211.153.80.29, is to visit the site IP address is 211.153.80.1, the domain name "www.ftedu.gov.cn". After commissioning, the unit where you can normally access the site. After returning home I try to ADSL access. Because ADSL uses PPPoE dial-up IP address changes without prior access can be normal, so I did not modify the settings in the company's notebook PC dial-up IP address on a hasty visit, SOHU, SINA no problem, QQ and MSN can also be normal login, 211.153.80.1 alone can not access the server company, there's a message that "page can not be displayed", the author used the domain name visit, fault still exists.
Troubleshooting
In order to find the source of the problem, I immediately called to colleagues at home through ADSL access test site, the results either by IP address or domain name, he can be a normal visit. Therefore, I will locate the root cause of failures in the local computer, and started investigation.
The first step: As the company uses a firewall, so Ping IP is unable to obtain feedback, so I use TRACERT command. At the command prompt type "tracert 211.153.80.1" and found immediately on the emergence of "Request timed out". This shows that the packets sent to 211.153.80.1 not reach the first routing device.
The second step: to check whether the DNS problems, on the command line mode, enter "nslookup", then enter "www.ftedu.gov.cn" and hit return, found that DNS server can correctly resolve the IP address "211.153.80.1 "illustrate the problem has nothing to do with DNS.
Step Three: Try the IP address of the neighboring track, that is, the command line mode, enter "tracert 211.153.81.1", the results can be found in the packet routing information through the five routing equipment.
Step Four: Enter the IPCONFIG command, I found the local computer's IP address is still set in the company's 211.153.80.29, subnet mask of 255.255.255.0. I suspect that is the cause of the failure to set production, once the IP address chosen automatically. After the failure to re-dial-up solution can be a normal visit to the home server unit.
Failure Analysis
Computer's IP address I will re-set to 211.153.80.29, the fault occurs again. In the command line mode, enter "route print" check this machine route, only to find the root cause of failure (see Figure). Originally set the local computer IP address 211.153.80.29, so visit 211.153.80.0 this segment, the system will direct the data packets sent to 211.153.80.29, and not sent to the default gateway address 61.51.199.192, which led to can not find the correct routing information.
Summary
With this failure, we get such an experience that should pay attention to the local IP settings, if the IP address of the local routing anomalies caused by improper settings, there will be access to certain segments can not normal phenomenon. We proposed such a failure in the event of cancellation of the unwanted use of IP.
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