Networking Ports Flashcards
And What They Do
Ports are a transport layer (Layer 4) concept.
Only a transport protocol such as the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) can indicate which port a packet should go to.
Port Numbers:
There are 65,535 possible port numbers, although not all are in common use.
The Internet Assigned Numbers Authority (IANA) maintains the full list of port numbers and protocols assigned to them.
TCP
TCP
Transmission Control Protocol
TCP is a standard that dictates how to establish and maintain a connection through which two programs may exchange data.
TCP’s job is to break down messages or files into smaller pieces (called packets) that are then sent over the Internet. These packets are then received by another TCP layer that then reassembles the data into a complete file or message. TCP is also responsible for error-checking that data stream to ensure the delivery of the data; if an error is found TCP retransmits the packet(s).
UDP
UDP
User Datagram Protocol
UDP is an alternative protocol to the (Transmission Control Protocol / Internet Protocol) TCP/IP that runs on top of IP and is sometimes referred to as UDP/IP. Unlike TCP/IP, UDP does not divide each transmission into packets, which allows for a faster transmission. However, UDP does not provide error checking.
PORT(s) 20/21
FTP
File Transfer Protocol
FTP is the most common way of sending and receiving files between two computers. An example of how FTP is used today is by web developers, who connect to their web server using an FTP client or FTP program (e.g., FileZilla) to send (upload) updated versions of a web page. Port 21 establishes connection and Port 20 transfers data.
PORT 22
SSH
Secure SHell
SSH (developed by SSH Communications Security Ltd.) is a secure protocol for remote logins. Using an SSH client, a user can connect to a server to transfer information in a more secure manner than other methods, such as telnet.
PORT 23
TELNET (TELetype NETwork)
A terminal emulation that enables a user to connect to a remote host or device using a telnet client, usually over port 23. For example, typing telnet hostname would connect a user to a hostname named hostname. Telnet enables a user to manage an account or device remotely. For example, a user may telnet into a computer that hosts their website to manage his or her files remotely. In the image to the right, is an example of a telnet session. As can be seen from this example, a telnet session is a command line interface.
PORT 25
SMTP
Simple Mail Transfer Protocol
SMTP is an Internet standard for the sending of e-mail messages over port 25. While it is mostly used for transfer from one mail server to another, some client mail applications use SMTP for relaying messages; whereas receiving happens via POP or IMAP.
PORT 53
DNS
Domain Name System
DNS is a service that receives a request containing a domain name hostname and responds with the corresponding IP address. The first DNS was designed and implemented by Paul Mockapetris and Jon Postel in 1983.
PORT(s) 67/68
DHCP
Dynamic Host Configuration Protocol
DHCP is a protocol used to assign an IP address to a computer or device connected to a network automatically. Routers, switches, or servers that use DHCP to assign addresses to other computerscan make setup and management easier for the network administrator.
On a home network, DHCP can be set up by purchasing a home router, wireless router, or switch with NAT/DHCP and connecting each computer to it. If the network has a firewall, ports 67 and 68 need to be open for devices to function properly.
PORT 80
HTTP
Hypertext Transfer Protocol
HTTP is a set of standards that allow users of the World Wide Web to exchange information found on web pages.
Today’s browsers no longer require HTTP in front of the URL since it is the default method of communication. However, it is kept in browsers because of the need to separate protocolssuch as FTP.
PORT 110
POP3
Post Office Protocol
POP or POP mail is one of the most commonly used protocols used to receive e-mail on many e-mail clients. There are two different versions of POP: POP2 and POP3.
POP2 was an early standard of POP that was only capable of receiving e-mail and required SMTP to send e-mail.
POP3 is the latest standard and can send and receive e-mail only using POP, but can also be used to receive e-mail and then use SMTP to send e-mail.
PORT 123
NTP
Network Time Protocol
The Network Time Protocol allows computer clocks to sync with each other, a process that is essential for encryption.
PORT(s) 137 to 139
NetBIOS/NetBT
Network Basic Input/Output System
NetBIOS was developed by IBM, Microsoft, and Sytek. NetBIOS allows computers and applications to communicate with network hardware, and allow data to be transmitted properly over a network. For example, Microsoft Windows computers that are named in a workgroup and not a domain use NetBIOS names, which are converted to IP addresses.
NetBIOS commonly communicates on ports 137, 138, and 139. If your firewall blocks any of these ports you may get errors when your computer communicates with another computer that utilizes NetBIOS (e.g., MicrosoftWindows). To prevent these errors or warnings, you must provide access on these ports.
Udp 137 nbname
Udp 138 nbdatagram
Tcp 139 nbsession
PORT 143
IMAP
Internet Message Access Protocol
IMAP was first developed by Mark Crispin while at Stanford University in 1986. It is a protocol for retrieving e-mail from a server, similar to POP. The secure version of IMAP is called IMAPS, which stands for IMAP over SSL.
PORT(s) 161/162
SNMP
Simple Network Management Protocol
SNMP was first introduced in 1988 and is a query, command, and response protocol.
SNMP examines and changes configuration parameters of LAN and WAN connected repeaters, bridges, routers, switches, and other devices connected to a network.
