what is a Port Number

                  A port is a communication point where one or more computers in a network communicate with each other through a program or software. There are about 0-65535 ports assigned to do specific tasks in a PC. Most ports operate with TCP or UDP protocol.

                Port numbers are assigned by an organisation called IANA  and ports are allocated to various needs to avoid confusion.

Ports are classified into 3 main categories..
    * Well Known Ports (Port numbers 0 - 1023)
    * Registered Ports (Port numbers1024 - 49151)
    * Private or Dynamic Ports (Port numbers 49152 - 65535)

Well Known Ports :
              Well known ports are used by system or processes run by root or with specific previleges. The port numbers range from 0 to 1023.

Registered Ports :
               The registered port numbers range from 1024-49151. Such ports are used by programs run by users in the system.

Private/Dynamic Ports :
               Private ports are not assigned for any specific purpose. its range are from range 49152–65535

some important port number :
  21 --- FTP
  23 --- Telnet
  25 --- SMTP
  53 --- DNS
  68 --- DHCP
  80 --- HTTP
110 --- POP3
119 --- NNTP
123 --- NTP (Network time protocol)
139 --- NetBIOS
161 --- SNMP
389 --- LDAP (Lightweight Directory Access Protocol)
443 --- HTTPS (HTTP over SSL/TLS)
443 --- SSL (https) (http protocol over TLS/SSL)
220 --- IMAP3
2049 --- TCP/UDP

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