This is bit important post on understanding the ports, their details and numbers.
Some FAQ related PORTS
Q. What is a port?
A port is software docking point in your machine where remote application uses for communication. This is analogy to the physical ports for entering in to the Countries.
Q. What is hardware port?
This is physical peripheral connection point to a machine from a physical device.
Q. What is a socket?
Socket is mix of software Port+IP address.
Q. What is the range of ports or how many ports are there?
Port numbers can vary from 0 to 65535, so total we can get 65536 ports
Q. Why port numbers are just 65536?
This is because limitation in TCP/IP stack where the port field is just 16bit size. So we get only 2^16 port which is equal to 65536 ports
Q.What are the well-known ports?
Well known ports are from 0 to 1023(total 2^10=1024 ports)
Q.What do you mean by default port?
Default port is a designated port for particular well-known server.
Q.Can we change default port for a service(example Apache, squid)?
Yes, we can change
Q.What are the protocol numbers for TCP and UDP?
TCP protocol number:6
UDP protocol number:17
Well known ports
20-FTP Data (For transferring FTP data) 21-FTP Control (For starting FTP connection) 22-SSH(For secure remote administration which uses SSL to encrypt the transmission) 23-Telnet (For insecure remote administration 25-SMTP(Mail Transfer Agent for e-mail server such as SEND mail) 53- DNS(Special service which uses both TCP and UDP) 67-Bootp 68-DHCP 69-TFTP(Trivial file transfer protocol uses udp protocol for connection less transmission of data) 80 -HTTP/WWW(apache) 88-Kerberos 110-POP3(Mail delivery Agent) 123-NTP(Network time protocol used for time syncing uses UDP protocol) 137-NetBIOS(nmbd) 139-SMB-Samba(smbd) 143-IMAP 161-SNMP(For network monitoring) 389-LDAP(For centralized administration) 443-HTTPS(HTTP+SSL for secure web access) 514-Syslogd(udp port) 995-POP3s 2049-NFS(nfsd, rpc.nfsd, rpc, portmap) 2401-CVS server 3306-MySql 6000-6063-X11
Note1:If protocol(TCP or UDP) is not mention then the above port are solely for TCP. Some service use UDP as mention in above list.
Note2:X11 use 6000 to 6063.. ports for connecting X11 from remote server