TCP/IP Troubleshooting

Ipconfig

IPConfig is a command-line tool that displays the current configuration of the installed IP stack on a networked computer.

When used with the /all switch, it displays a detailed configuration report for all interfaces, including any configured WAN miniports (typically used for remote access or VPN connections). Output can be redirected to a file and pasted into other documents. A sample report is shown here:

C:>\ipconfig /all

Windows 2000 IP Configuration

        Host Name . . . . . . . . . . . . : TESTPC1

        Primary DNS Suffix  . . . . . . . : reskit.com

        Node Type . . . . . . . . . . . . : Hybrid

        IP Routing Enabled. . . . . . . . : No

        WINS Proxy Enabled. . . . . . . . : No

DNS Suffix Search List. . . . . . : ntcorpdc1.reskit.com

        dns.reskit.com

        reskit.com

Ethernet adapter Local Area Connection:

        Connection-specific DNS Suffix  . : dns.reskit.com

        Description . . . . . . . . . . . : Acme XL 10/100Mb Ethernet NIC

        Physical Address. . . . . . . . . : 00-CC-44-79-C3-AA

        DHCP Enabled. . . . . . . . . . . : Yes

        IP Address. . . . . . . . . . . . : 172.16.245.111

        Subnet Mask . . . . . . . . . . . : 255.255.248.0

        Default Gateway . . . . . . . . . : 172.16.240.1

        DHCP Server . . . . . . . . . . . : 172.16.248.8

        DNS Servers . . . . . . . . . . . : 172.16.55.85

                                            172.16.55.134

                                            172.16.55.54

        Primary WINS Server . . . . . . . : 172.16.248.10

        Secondary WINS Server . . . . . . : 172.16.248.9

        Lease Obtained. . . . . . . . . . : Friday, May 05, 1999 2:21:40 PM

        Lease Expires . . . . . . . . . . : Monday, May 07, 1999 2:21:40 PM

A number of other useful parameters for Ipconfig include /flushdns, which deletes the DNS name cache; /registerdns, which refreshes all DHCP leases and re-registers DNS names; and /displaydns which displays the contents of the DNS resolver cache.

The /release <adapter> and /renew <adapter> options release and renew the DHCP-allocated IP address for a specified adapter. If no adapter name is specified, the DHCP leases for all adapters bound to TCP/IP are released or renewed.

For /setclassid, if no class ID is specified, then the Class ID is removed. Table 3.3 lists all Ipconfig switches.

Table 3.3 Ipconfig Switches

Switch	Effect
/all	Produces a detailed configuration report for all interfaces.
/flushdns	Removes all entries from the DNS name cache.
/registerdns	Refreshes all DHCP leases and reregisters DNS names
/displaydns	Displays the contents of the DNS resolver cache.
/release <adapter>	Releases the IP address for a specified interface.
/renew <adapter>	Renews the IP address for a specified interface.
/showclassid <adapter>	Displays all the DHCP class IDs allowed for the adapter specified.
/setclassid <adapter> <classID to set>	Changes the DHCP class ID for the adapter specified.
/?	Displays this list.

The /showclassid and /setclassid options allow you to manipulate user class IDs from the command line. The user class IDs are options that a system administrator may set on the DHCP server to configure a client computer to identify itself with the server. Issuing the command ipconfig /showclassid <adapter> sends a query to the client's server; the server responds by providing the available classes. Once you know which classes are available, you can issue a command like ipconfig /setdhcpclassid <adapter> <class ID to set on the server> to set the class ID that the client will use from that point on. For more information about DHCP and class IDs, see "Dynamic Host Configuration Protocol" in this book.