Important Port Numbers

This is very important questions mostly asked during Linux Interview.

I have collected some ports info to understanding the ports, their
details and numbers.

Well known common 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

6000-6063-X11

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.

Other Linux Topic Interview Questions…