Windows7: Troubleshooting Networking from the Command Line (part 1)

1/5/2011 2:41:00 PM

Windows 7 TCP/IP comes with a few command-line utilities that you can use to review your TCP/IP settings and troubleshoot problems. Here’s a list of the available utilities:

ARP	This utility displays (or modifies) the IP-to-Ethernet or IP-to-Token Ring address translation tables used by the Address Resolution Protocol (ARP) in TCP/IP. Enter the command arp -? for the syntax.
NBTSTAT	This utility displays the protocol statistics and the current TCP/IP connections using NBT (NetBIOS over TCP/IP). Enter nbtstat -? for the syntax.
NETSTAT	This utility displays the protocol statistics and current TCP/IP connections. The command netstat -? displays the syntax.
PING	This utility can check a network connection to a remote computer. This is one of the most commonly used TCP/IP diagnostic tools, so I describe it more detail in the next section.
ROUTE	This utility can manipulate a network routing table (LMHOSTS). Enter route -? for the syntax.
TRACERT	This utility can check the route taken to a remote host. I explain this valuable diagnostic command in more detail later.
IPCONFIG	This utility displays the current TCP/IP network configuration. If you run the command ipconfig without any switches, the utility returns your system’s current IP address, subnet mask, and default gateway. If you run the command ipconfig /all, the utility returns more details information, as shown here:

Windows IP Configuration

 Host Name . . . . . . . . . . . . : MediaPC
 Primary Dns Suffix  . . . . . . . :
 Node Type . . . . . . . . . . . . : Hybrid
 IP Routing Enabled. . . . . . . . : No
 WINS Proxy Enabled. . . . . . . . : No
 DNS Suffix Search List. . . . . . : phub.net.cable.window.com

Ethernet adapter Local Area Connection:

 Connection-specific DNS Suffix  . : phub.net.cable.rogers.com
 Description . . . . . . . . . . . : Atheros L1 Gigabit Ethernet 10/100/1000Base-T Controller
 Physical Address. . . . . . . . . : 00-1E-8C-7D-97-3A
 DHCP Enabled. . . . . . . . . . . : Yes
 Autoconfiguration Enabled . . . . : Yes
 Link-local IPv6 Address . . . . . : fe80::452f:6db7:eaf2:3112%11(Preferred)
 IPv4 Address. . . . . . . . . . . : 192.168.0.84(Preferred)
 Subnet Mask . . . . . . . . . . . : 255.255.0.0
 Lease Obtained. . . . . . . . . . : Wednesday, April 15, 2009 10:38:42 AM
 Lease Expires . . . . . . . . . . : Thursday, April 16, 2009 2:07:16 PM
 Default Gateway . . . . . . . . . : 192.168.1.1
 DHCP Server . . . . . . . . . . . : 192.168.1.1
 DHCPv6 IAID . . . . . . . . . . . : 234888844
 DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-11-52-C4-05-00-1E-8C-7D-97-3A
 DNS Servers . . . . . . . . . . . : 192.168.1.1
 NetBIOS over Tcpip. . . . . . . . : Enabled

Tunnel adapter isatap.phub.net.cable.rogers.com:

 Media State . . . . . . . . . . . : Media disconnected
 Connection-specific DNS Suffix  . : phub.net.cable.rogers.com
 Description . . . . . . . . . . . : Microsoft ISATAP Adapter
 Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
 DHCP Enabled. . . . . . . . . . . : No
 Autoconfiguration Enabled . . . . : Yes

Tunnel adapter Teredo Tunneling Pseudo-Interface:

 Connection-specific DNS Suffix  . :
 Description . . . . . . . . . . . : Teredo Tunneling Pseudo-Interface
 Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
 DHCP Enabled. . . . . . . . . . . : No
 Autoconfiguration Enabled . . . . : Yes
 IPv6 Address. . . . . . . . . . . : 2001:0:4137:9e50:3032:38c1:3f57:ffab(Preferred)
 Link-local IPv6 Address . . . . . : fe80::3032:38c1:3f57:ffab%13(Preferred)
 Default Gateway . . . . . . . . . : ::
 NetBIOS over Tcpip. . . . . . . . : Disabled

Tunnel adapter Reusable ISATAP Interface {D767BCA8-D27E-404C-9A50-CD680EF507C0}:

 Media State . . . . . . . . . . . : Media disconnected
 Connection-specific DNS Suffix  . :
 Description . . . . . . . . . . . : Microsoft ISATAP Adapter #2
 Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
 DHCP Enabled. . . . . . . . . . . : No
 Autoconfiguration Enabled . . . . : Yes

A Basic Command-Line Troubleshooting Procedure

Here’s a basic procedure you can run through to troubleshoot networking problems using the command-line tools:

1.	Release the current DHCP lease by running the following command: ipconfig /release
2.	Renew the DNCP lease by running the following command: ipconfig /renew Note A DHCP lease is a guarantee that the Dynamic Host Control Protocol (DHCP) client computer will have the IP address supplied by the DHCP server for a specified period of time. To avoid lease expiration, the DHCP client usually sends a request—a DHCPREQUEST message—for lease renewal to the original DHCP server after 50% of the lease time has expired. If 87.5% of its lease time has expired, the DHCP client it sends a lease renewal request to all available DHCP servers.
3.	Flush the ARP cache. The ARP handles the conversion of an IP address to a physical address of a network adapter. (To see the physical address of your adapter, open the connection’s Status dialog box, display the Support tab, and click Details.) To improve performance, Windows 7 stores resolved addresses in the ARP cache for a short time. Some networking problems are caused by ARP cache entries that are obsolete or incomplete. The cache is normally flushed regularly, but to force a flush, run the following command: arp -d Tip To see the contents of the ARP cache, run the following command: arp -a You’ll see output similar to the following: Interface: 192.168.1.101 —- 0x2 Internet Address Physical Address Type 192.168.1.1 00-12-17-8c-48-88 dynamic 192.168.1.100 00-11-24-1a-7a-fc dynamic 192.168.1.103 00-11-11-be-c7-78 dynamic
4.	Flush the NetBIOS name cache. NetBIOS handles the conversion between the network names of computers and their IP addresses. To improve performance, Windows 7 stores resolved names in the NetBIOS name cache. To solve problems caused by NetBIOS name cache entries that are obsolete or bad, this step clears the cache. Run the following command: nbtstat -r
5.	Re-register the computer with the network’s WINS server. That is, you ask the WINS server to release the computer’s NetBIOS names that are registered with the server and then re-register them. This is useful if you’re having problems connecting to other computers using their network names. Run the following command: nbtstat -rr
6.	Flush the DNS cache. DNS handles the conversion of domain names to IP addresses. To improve performance, Windows 7 stores resolved domain names in the DNS cache. To solve problems caused by DNS cache entries that are obsolete or bad, clear the cache by running the following command: ipconfig /flushdns
7.	Re-register the computer with the DNS server. This is useful if you’re having trouble resolving domain names or if you’re having trouble with a dynamic DNS server. Run the following command: ipconfig /registerdns