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ipconfig



ipconfig is a commmand line utility available on all versions of Microsoft Windows starting with Windows NT. ipconfig is designed to be run from the Windows command prompt. This utility allows you to get the IP address information of a Windows computer. It also allows some control over active TCP/IP connections. ipconfig is an alternative to the older 'winipcfg' utility.

ipconfig Usage

From the command prompt, type 'ipconfig' to run the utility with default options. The output of the default command contains the IP address, network mask and gateway for all physical and virtual network adapters.
'ipconfig' supports several command line options as described below. The command "ipconfig /?" displays the set of available options.

ipconfig /all

This option displays the same IP addressing information for each adapter as the default option. Additionally, it displays DNS and WINS settings for each adapter.

ipconfig /release

This option terminates any active TCP/IP connections on all network adapters and releases those IP addresses for use by other applications. 'ipconfig /release" can be used with specific Windows connection names. In this case, the command will affect only the specified connections and not all. The command accepts either full connection names or wildcard names. Examples:
    ipconfig /release "Local Area Connection 1"
    ipconfig /release *Local*

ipconfig /renew

This option re-establishes TCP/IP connections on all network adapters. As with the release option, ipconfig /renew takes an optional connection name specifier.
Both /renew and /release options only work on clients configured for dynamic (DHCP) addressing.
Note: The remaining options below are only available on Windows 2000 and newer versions of Windows.

ipconfig /showclassid, ipconfig /setclassid

These options manage DHCP class identifiers. DHCP classes can be defined by administrators on a DHCP server to apply different network settings to different types of clients. This is an advanced feature of DHCP typically used in business networks, not home networks.

ipconfig /displaydns, ipconfig /flushdns

These options access a local DNS cache that Windows maintains. The /displaydns option prints the contents of the cache, and the /flushdns option erases the contents.
This DNS cache contains a list of remote server names and the IP addresses (if any) they correspond to. Entries in this cache come from DNS lookups that happen when attempting to visit Web sites, named FTP servers, and other remote hosts. Windows uses this cache to improve the performance of Internet Explorer and other Web-based applications.
In home networking, these DNS options are sometimes useful for advanced troubleshooting. If the information in your DNS cache becomes corrupted or outdated, you could face difficulty accessing certain sites on the Internet. Consider these two scenarios:
  • The IP address of a Web site, email server or other server changes (rare occurence). The name and address of this site normally stay in your cache for 24 hours after your last visit. You may need to clear your cache to access the server sooner.
  • A Web site or other server was offline when you last visited it (hopefully a rare occurence) but since has come back online. The cache will normally keep a record that the server is offline for 5 minutes afer your last visit. You may need to clear your cache to access the server sooner.

ipconfig /registerdns

Similar to the above options, this option updates DNS settings on the Windows computer. Instead of merely accessing the local DNS cache, however, this option initiates communication with both the DNS server (and the DHCP server) to re-register with them.
This option is useful in troubleshooting problems involving connection with the Internet service provider, such as failure to obtain a dynamic IP address or failure to connect to the ISP DNS server.
Like the /release and /renew options, /registerdns optionally takes the name(s) of specific adapters to update. If no name parameter is specified, /registerdns updates all adapter

Basic Telnet Tutorial

Telnet was developed in 1969 to aid in remote connectivity between computers over a network. Telnet can connect to a remote machine that on a network and is port listening. Most common ports to which one can connect to through telnet are:
Port 21 - File Transfer Protocol
Port 22 - SSH Remote Login Protocol
Port 23 - Telnet Server
Port 25 - Simple Mail Transfer Protocol (SMTP)
Port 53 - Domain Name Server (DNS)
Port 69 - Trivial File Transfer Protocol (TFTP)
Port 70 - Gopher
Port 80 - Hyper Text Transfer Protocol (HTTP)
Port 110 - Post Office Protocol 3 (POP3)

Telnet can be used to connect to virtually any machine that listens on ports. In other words, you can connect to any machine that has certain ports open. Once connected to a machine, you need to issue unix based commands to interact with the remote service. For example, a user don't need to login, check and send mails only through his email service provider's interface but this can be achieved using simple telnet commands. It is because of this reason that many hackers can send spoofed emails or access information such as which services are running on the remote machine. This is also called banner grabbing or daemon tracking. Black hat hackers can also use telnet to sniff network packets which might contain sensitive information such as usernames and passwords. This is achieved by using telnet and network utilities such as tcpdump and wireshark.

Telnet client and server functionality comes built-in in most operating systems. However, there are several third-party applications like putty client that enable remote connectivity. A user can connect to a remote machine through several access modes such as raw access, SSH access, etc. SSH mode offers encryption and security and hence can prevent eavesdropping by hackers. This is by far the most secure way of connecting to a machine. However, it is necessary that the remote machine supports SSH login to make use of the encryption and security features.

On windows machines, telnet client can simply be started by issuing the telnet command in windows command shell. The following example would help you connect to a remote machine on the HTTP Port 80 and issue a GET command which would fetch a file as your web browser does it behind scenes:

Command Prompt> Telnet
Command Prompt> open (somedomain.com or ip address) 80

At this stage, you would be connected to somedomain.com or ip address on port 80 and the daemon that is running on port 80 (most probably HTTP Server) would be waiting for HTTP requests.

GET / HTTP/1.1
host: somedomain.com (or ip address)
Issuing the command above would make the HTTP Server return the file requested, in this case it would be the default file at the root location. You can find some additional basic telnet commands at the Microsoft Technet library -http://technet.microsoft.com/en-us/library/c.aspx
Most applications and embedded devices make use of the telnet technology to connect to remote server machines and provide end user functionality. The most common use of telnet stands to enable remote authentication and access.