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Dated: Aug. 13, 2004

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By Najmi

TCP/IP (Transmission Control Protocol/Internet Protocol), the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols, such as Netware, also support TCP/IP.

UDP(User Datagram Protocol) a connectionless protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagrams over an IP network. It's used primarily for broadcasting messages over a network.

In TCP/IP and UDP networks, a port is an endpoint to a logical connection and the way a client program specifies a specific server program on a computer in a network. Some ports have numbers that are preassigned to them by the IANA, and these are known as well-known ports (specified in RFC 1700). IANA Internet Assigned Numbers Authority, an organisation working under the auspices of the Internet Architecture Board (IAB) that is responsible for assigning new Internet-wide IP addresses Port numbers range from 0 to 65536, but only ports numbers 0 to 1024 are reserved for privileged services and designated as well-known ports. This list of well-known port numbers specifies the port used by the server process as its contact port.

Port Number Description

1 TCP Port Service Multiplexer (TCPMUX)

5 Remote Job Entry (RJE) 


18 Message Send Protocol (MSP) 

20 FTP – Data. File Transfer Protocol is a protocol used on the Internet for sending files.

21 FTP -- Control 

22 SSH Remote Login Protocol. Developed by SSH Communications Security Ltd., Secure Shell is a program to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine to another. It provides strong authentication and secure communications over insecure channels. It is a replacement for rlogin, rsh, rcp, and rdist. 

SSH protects a network from attacks such as IP spoofing, IP source routing, and DNS spoofing. An attacker who has managed to take over a network can only force ssh to disconnect. He or she cannot play back the traffic or hijack the connection when encryption is enabled. 

When using ssh's slogin (instead of rlogin) the entire login session, including transmission of password, is encrypted; therefore it is almost impossible for an outsider to collect passwords. 

SSH is available for Windows, Unix, Macintosh, and OS/2, and it also works with RSA authentication. 

23 Telnet. A terminal emulation program for TCP/IP networks such as the Internet. The Telnet program runs on your computer and connects your PC to a server on the network. You can then enter commands through the Telnet program and they will be executed as if you were entering them directly on the server console. This enables you to control the server and communicate with other servers on the network. To start a Telnet session, you must log in to a server by entering a valid username and password. Telnet is a common way to remotely control Web servers.

25 Simple Mail Transfer Protocol (SMTP). Short for Simple Mail Transfer Protocol, a protocol for sending e-mail messages between servers. Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.

TCP Ports

29  MSG ICP 

37  Time 

42  Host Name Server (Nameserv) 

43  WhoIs 

49  Login Host Protocol (Login) 

53 Domain Name System (DNS). Short for Domain Name System (or Service), an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name might translate to 

The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned. 

69 Trivial File Transfer Protocol (TFTP). Trivial File Transfer Protocol, a simple form of the File Transfer Protocol (FTP). TFTP uses the User Datagram Protocol (UDP)and provides no security features. It is often used by servers to boot diskless workstations, X-terminals, and routers.

70 Gopher Services . A system that pre-dates the World Wide Web for organising and displaying files on Internet servers. A Gopher server presents its contents as a hierarchically structured list of files. With the ascendance of the Web, many gopher databases were converted to Web sites which can be more easily accessed via Web search engines. 

Gopher was developed at the University of Minnesota and named after the school's mascot. Two systems, Veronica and Jughead, let you search global indices of resources stored in Gopher systems. 

79 Finger. A UNIX program that takes an e-mail address as input and returns information about the user who owns that e-mail address. On some systems, finger only reports whether the user is currently logged on. Other systems return additional information, such as the user's full name, address, and telephone number. Of course, the user must first enter this information into the system. Many e-mail programs now have a finger utility built into them. 

80 HTTP. Short for HyperText Transfer Protocol, the underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. For example, when you enter a URL in your browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page. 

The other main standard that controls how the World Wide Web works is HTML, which covers how Web pages are formatted and displayed. 

HTTP is called a stateless protocol because each command is executed independently, without any knowledge of the commands that came before it. This is the main reason that it is difficult to implement Web sites that react intelligently to user input. This shortcoming of HTTP is being addressed in a number of new technologies, including ActiveX, Java, JavaScript and cookies. 

103  X.400 Standard. An ISO and ITU standard for addressing and transporting e-mail messages. It conforms to layer 7 of the OSI model and supports several types of transport mechanisms, including Ethernet, X.25, TCP/IP, and dial-up lines.

108  SNA Gateway Access Server.

109  POP2

110  POP3 

115  Simple File Transfer Protocol (SFTP) 

118  SQL Services 

119  Newsgroup (NNTP) 

137  NetBIOS Name Service. Network Basic Input Output System, an application programming interface (API) that augments the DOS BIOS by adding special functions for local-area networks (LANs). Almost all LANs for PCs are based on the NetBIOS. Some LAN manufacturers have even extended it, adding additional network capabilities.

139  NetBIOS Datagram Service 

143  Interim Mail Access Protocol (IMAP) 

150  NetBIOS Session Service 

156  SQL Server 

161  SNMP 

179  Border Gateway Protocol (BGP) Border Gateway Protocol, an exterior gateway routing protocol that enables groups of routers (called autonomous systems) to share routing information so that efficient, loop-free routes can be established. BGP is commonly used within and between Internet Service Providers (ISPs). The protocol is defined in RFC 1771. 

190  Gateway Access Control Protocol (GACP) 

194  Internet Relay Chat (IRC) 

197  Directory Location Service (DLS) 

389  Lightweight Directory Access Protocol (LDAP) 

396  Novell Netware over IP 

443  HTTPS 

444  Simple Network Paging Protocol (SNPP) 

445  Microsoft-DS 

458  Apple QuickTime 

546  DHCP Client Short for Dynamic Host Configuration Protocol, a protocol for assigning dynamic IP addresses to devices on a network. With dynamic addressing, a device can have a different IP address every time it connects to the network. In some systems, the device's IP address can even change while it is still connected. DHCP also supports a mix of static and dynamic IP addresses. 

Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than requiring an administrator to manage the task. This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address. Many ISPs use dynamic IP addressing for dial-up users. 

DHCP client support is built into Windows 95 and NT workstation. NT 4 server includes both client and server support. 

547 DHCP Server 

563 SNEWS 

9 MSN 

1080 Socks 

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04 Tue Oct 2011
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discounted cheap laptop computers's Comment
Indeed a very good read! Very informative post with pretty good insight on all aspects of the topic! Will keep visiting in future too!
20 Wed Jul 2011
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khan's Comment
Excellent work
06 Mon Sep 2010
Admin's Reply:

Thanks Khan.

Amit's Comment
This is Very good knowledge to know the computer world by this site.
27 Sun Jun 2010
Admin's Reply:

I really appreciate your comment Amit .

Ollie's Comment
Well I've tried connecting to my router through port 23 but it's not working (I'm new to telnet)
08 Fri Jan 2010
Admin's Reply:

Off course, I'm not well aware of your situaltion or setup. But Telnetting to your router didn't make sense to me. I however, not a TELNET expert by any means.

But you have a server setup with a TELNET service installed, try it on that. Or better yet, look up a Free TELNET account online and practice you're skills on it.

I'm not sure if I answered your question, but it just poped in my head

Ollie's Comment
I have tried many of these and I get the same message "could not open connection to host, on port x". Can you help me?
07 Thu Jan 2010
Admin's Reply:

where exactly are you trying?

Tarek's Comment
04 Mon Jan 2010
Admin's Reply:

Thanks for stopping by Tarek.