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The five-layer TCP/IP model
5 Script Hyperterminal. .  Application layer

DHCP · DNS · FTP · Gopher · HTTP · IMAP4 · IRC · NNTP · XMPP · POP3 · SIP · SMTP · SNMP · SSH · TELNET · RPC · RTCP · RTSP · TLS · SDP · SOAP · GTP · STUN · NTP · RIP · Script Hyperterminal. Script Hyperterminal. Script Hyperterminal.
4 Script Hyperterminal. Transport layer

TCP · UDP · DCCP · SCTP · RTP · RSVP · IGMP · ICMP · ICMPv6 · PPTP · Script Hyperterminal. Script Hyperterminal. Script Hyperterminal.
3 Script Hyperterminal. Network/Internet layer

IP (IPv4 · IPv6) · OSPF · IS-IS · BGP · IPsec · ARP · RARP · Script Hyperterminal. Script Hyperterminal. Script Hyperterminal.
2 Script Hyperterminal. Data link layer

802 Script Hyperterminal.11 · Wi-Fi · WiMAX · ATM · DTM · Token Ring · Ethernet · FDDI · Frame Relay · GPRS · EVDO · HSPA · HDLC · PPP · L2TP · ISDN · Script Hyperterminal. Script Hyperterminal. Script Hyperterminal.
1 Script Hyperterminal. Physical layer

Ethernet physical layer · Modems · PLC · SONET/SDH · G Script Hyperterminal.709 · OFDM · Optical Fiber · Coaxial Cable · Twisted Pair · Script Hyperterminal. Script Hyperterminal. Script Hyperterminal.

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For the packet switched network, see Telenet Script Hyperterminal.
TELNET (TELecommunication NETwork) is a network protocol used on the Internet or local area network (LAN) connections Script Hyperterminal. It was developed in 1969 beginning with RFC 15 and standardized as IETF STD 8, one of the first Internet standards Script Hyperterminal.
The term telnet also refers to software which implements the client part of the protocol Script Hyperterminal. TELNET clients have been available on most Unix systems for many years and are available for virtually all platforms Script Hyperterminal. Most network equipment and OSs with a TCP/IP stack support some kind of TELNET service server for their remote configuration (including ones based on Windows NT) Script Hyperterminal. Recently, Secure Shell has begun to dominate remote access for Unix-based machines Script Hyperterminal.
“To telnet” is also used as a verb meaning to establish or use a TELNET or other interactive TCP connection, as in, “To change your password, telnet to the server and run the passwd command” Script Hyperterminal.
Most often, a user will be telneting to a Unix-like server system or a simple network device such as a switch Script Hyperterminal. For example, a user might “telnet in from home to check his mail at school” Script Hyperterminal. In doing so, he would be using a telnet client to connect from his computer to one of his servers Script Hyperterminal. Once the connection is established, he would then log in with his account information and execute operating system commands remotely on that computer, such as ls or cd Script Hyperterminal.
On many systems, the client may also be used to make interactive raw-TCP sessions, even when that option is not available, telnet sessions are equivalent to raw TCP as long as byte 255 never appears in the data Script Hyperterminal.

Protocol details

TELNET is a client-server protocol, based on a reliable connection-oriented transport Script Hyperterminal. Typically this is TCP port 23, although TELNET predates TCP/IP and was originally run on NCP Script Hyperterminal.
The protocol has many extensions, some of which have been adopted as Internet standards Script Hyperterminal. IETF standards STD 27 through STD 32 define various extensions, most of which are extremely common Script Hyperterminal. Other extensions are on the IETF standards track as proposed standards Script Hyperterminal.

Security

When TELNET was initially developed in 1969, most users of networked computers were in the computer departments of academic institutions, or at large private and government research facilities Script Hyperterminal. In this environment, security was not nearly as much of a concern as it became after the bandwidth explosion of the 1990s Script Hyperterminal. The rise in the number of people with access to the Internet, and by extension, the number of people attempting to crack other people”s servers made encrypted alternatives much more necessary Script Hyperterminal.
Experts in computer security, such as SANS Institute, and the members of the comp Script Hyperterminal.os Script Hyperterminal.linux Script Hyperterminal.security newsgroup recommend that the use of TELNET for remote logins should be discontinued under all normal circumstances, for the following reasons:

• TELNET, by default, does not encrypt any data sent over the connection (including passwords), and so it is often practical to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, switch, or gateway located on the network between the two hosts where TELNET is being used can intercept the packets passing by and obtain login and password information (and whatever else is typed) with any of several common utilities like tcpdump and Wireshark Script Hyperterminal.
• Most implementations of TELNET lack an authentication scheme that makes it possible to ensure that communication is carried out between the two desired hosts, and not intercepted in the middle Script Hyperterminal.
• Commonly used TELNET daemons have several vulnerabilities discovered over the years Script Hyperterminal.

These security-related shortcomings have seen the usage of the TELNET protocol drop rapidly, especially on the public Internet, in favor of a the ssh protocol, first released in 1995 Script Hyperterminal. SSH provides much of the functionality of telnet, with the addition of strong encryption to prevent sensitive data such as passwords from being intercepted, and public key authentication, to ensure that the remote computer is actually who it claims to be Script Hyperterminal.
As has happened with other early Internet protocols, extensions to the TELNET protocol provide TLS security and SASL authentication that address the above issues Script Hyperterminal. However, most TELNET implementations do not support these extensions; and there has been relatively little interest in implementing these as SSH is adequate for most purposes Script Hyperterminal. The main advantage of TLS-TELNET would be the ability to use certificate-authority signed server certificates to authenticate a server host to a client that does not yet have the server key stored Script Hyperterminal. In SSH, there is a weakness in that the user must trust the first session to a host when it has not yet acquired the server key Script Hyperterminal.
Clients and servers designed to pass IBM 5250 data streams over Telnet generally do support SSL encryption, as SSH does not include 5250 emulation Script Hyperterminal. Under OS/400, Port 992 is the default port for Secured Telnet Script Hyperterminal.

Current status

As of the mid-2000s, while the TELNET protocol itself has been mostly superseded, TELNET clients are still used, often when diagnosing problems, to manually “talk” to other services without specialized client software Script Hyperterminal. For example, it is sometimes used in debugging network services such as an SMTP, IRC or HTTP server, by serving as a simple way to send commands to the server and examine the responses Script Hyperterminal.
However, other software such as nc (netcat) or socat on Unix (or PuTTY on Windows) are finding greater favor with some system administrators for testing purposes, as they can be called with arguments not to send any terminal control handshaking data Script Hyperterminal. Also netcat does not distort the \377 octet, which allows raw access to TCP socket, unlike any standard-compliant TELNET software Script Hyperterminal.
TELNET is still very popular in enterprise networks to access host applications, e Script Hyperterminal.g Script Hyperterminal. on IBM Mainframes Script Hyperterminal.
TELNET is still widely used for administration of network elements, e Script Hyperterminal.g Script Hyperterminal., in commissioning, integration and maintenance of core network elements in mobile communication networks Script Hyperterminal.
TELNET is also heavily used for MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the resurgent BBS community Script Hyperterminal.
In the 2007 Microsoft Windows release, Windows Vista, Telnet Script Hyperterminal.exe is no longer installed by default, but is still included as an installable feature Script Hyperterminal.

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October 20th 2007 Posted to   Pills