The “Creating an End-System Network Configuration Table” section earlier in this chapter
introduced the ipconfig command used with the /all option. While this option is useful for
gathering information on a system, other options in the ipconfig command are helpful
for troubleshooting purposes.
The first of these options are the /release and /renew options, which are used to release
and renew DHCP addresses. Here are two examples:
C:\>ipconfig /release
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 0.0.0.0
Subnet Mask . . . . . . . . . . . : 0.0.0.0
Default Gateway . . . . . . . . . :
C:\WINDOWS\system32>ipconfig /renew
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 10.22.5.3
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 10.22.5.1
The next option useful in troubleshooting is the /displaydns option. It allows you to see the
DNS-name-to-IP-address cache that is on the workstation. The following output is from an XP
machine right after pinging www.cisco.com:
C:\>ipconfig /displaydns
Windows IP Configuration
ns1.cisco.com
----------------------------------------
Record Name . . . . . : ns1.cisco.com
Record Type . . . . . : 1
Time To Live . . . . : 86227
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 128.107.241.185
1.0.0.127.in-addr.arpa
----------------------------------------
Record Name . . . . . : 1.0.0.127.in-addr.arpa.
Record Type . . . . . : 12
Time To Live . . . . : 0
Data Length . . . . . : 4
Section . . . . . . . : Answer
PTR Record . . . . . : localhost
ns2.cisco.com
----------------------------------------
Record Name . . . . . : ns2.cisco.com
Record Type . . . . . : 1
Time To Live . . . . : 86227
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 192.135.250.69
www.cisco.com
----------------------------------------
Record Name . . . . . : www.cisco.com
Record Type . . . . . : 1
Time To Live . . . . : 86227
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 198.133.219.25
Record Name . . . . . : ns1.cisco.com
Record Type . . . . . : 1
Time To Live . . . . : 86227
Data Length . . . . . : 4
Section . . . . . . . : Additional
A (Host) Record . . . : 128.107.241.185
Record Name . . . . . : ns2.cisco.com
Record Type . . . . . : 1
Time To Live . . . . : 86227
Data Length . . . . . : 4
Section . . . . . . . : Additional
A (Host) Record . . . : 192.135.250.69
localhost
----------------------------------------
Record Name . . . . . : localhost
Record Type . . . . . : 1
Time To Live . . . . : 0
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 127.0.0.1
In addition to the local DNS cache on the machine, Internet Explorer (IE) keeps its
own name resolution cache. By default, names are cached in Internet Explorer
versions 4.0 or higher for 30 minutes, and for 24 hours in IE versions below 4.0.
Shutting down Internet Explorer and restarting it will refresh this cache.
The final option for the ipconfig command that we will discuss is /flushdns, which is complementary
to the /displaydns option. The /flushdns option clears out all entries in the DNS
cache on the workstation. This works out well for troubleshooting stale DNS entries. The output
of the command is shown here:
C:\>ipconfig /flushdns
Windows IP Configuration
Successfully flushed the DNS Resolver Cache.
C:\>