This is an important question mostly asked during Linux Interview.
To understanding the ports, their details and numbers, I have collected some info.
FAQ related to PORTS in Linux !!
Q. What is a port?
A port is piece of software which is used as docking point in your machine, where remote application can communicate. This is analogy to the physical ports for entering in to a country from different sea ports.
Q. What is hardware port?
This is a physical peripheral connecting point to a machine from a physical device.
Q. What is a socket?
Socket is combination of software Port and 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 number field is just 16bit size. So we get only 2^16(2 to the power of 16) ports which are equal to 65536 available ports
Q.What are the well-known ports or assigned ports or default 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 service such as web server, mail server, ftp server etc. By default FTP uses 21 port, DNS uses 53 and Apache uses 80 port.
Q.Can we change default port for a service(example Apache, squid)?
Yes, we can change. In Apache and DNS we can change this using listen configuration entry in httpd.conf and named.conf. Squid have port entry in its squid.conf file to mention port number.
Q.What are the protocol numbers for TCP and UDP?
Do not confuse this one with port numbers. TCP and UDP have their own numbers in TCP/IP stack.
TCP protocol number:6
UDP protocol number:17
Q. Is there any way I can see all the port information in Linux?
Yes, you can get that from /etc/services files.
Q. How can I see open ports in Linux?
Use nmap command.
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)
636 – ldaps(both tcp and udp)
873 – rsync
989 – FTPS-data
990 – FTPS
993 – IMAPS
1194 – openVPN
1812 – RADIUS
995 – POP3s
2049 – NFS(nfsd, rpc.nfsd, rpc, portmap)
2401 – CVS server
3306 – MySql
3690 – SVN
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.