LAB 3 - PORTS

PORT
Logical (not physical) connection to computer (server).

One hardware link: Many ports.

One host (physical server) can run many services (listening processes) at different addresses.

IP address = Address of a host.

IP address + port = Address of a process (service) on a host.

List of ports. 1 to 65535 (16 bit no). 

List of ports

Ports set aside for some "well-known" services:

20 - ftp data

21 - ftp control (commands)

22 - ssh / sftp / scp

23 - telnet

25 - SMTP - standard for email transmission from node to node

53 - DNS

80 - http

110 - POP3 - retrieve email to local client

143 - IMAP - more advanced email client

443 - https (http over SSL/TLS)

554 - rtsp

989 - ftps (ftp over SSL/TLS) (data)

990 - ftps (control)

1935 - rtmp

8080 - http alternate

Full list:

• Internet Assigned Numbers Authority 
  
  port numbers
• Server machine may run multiple server processes, each contactable on different port. 
• Conversely, multiple clients may want to contact same port (e.g. Web server).
• Client creates socket at its end. Sends request to server (at port no). Server creates socket at its end dedicated to that client. 
• One port: Many sockets to that port.
• Port scanner - look at some machines in DCU to find ports that are "open" - providing a service.
• Does this by trying to open a socket to that port.

PROGRAM

import java.net.*;
import java.io.*;

public class ports
{
public static void main (String[] args) throws IOException
{
String hostname = args[0];
Socket s = null;
InetAddress ipaddress = InetAddress.getByName(hostname);
int p=80;
s = new Socket(hostname, p);
System.out.println("A server is running on port " + p + ".");
s.close();
}
}

OUTPUT

G:\>javac ports.java
G:\> java ports www.dbu. edu.et

A server is running on port 80.

note: Search for IMAP servers. And POP3 servers:.