Network Basics: Services Port Assignment
The port numbers from 0 to 1023 are called well-known ports. They are used by system processes that provide widely-used network services. So what services use what ports in the world of TCP and UDP? Here is a short list (from the hosts file on a standard computer) of some of the major services in the well-known ports registered range.
You may notice that some of the ports are used by the services on both UDP and TCP, but this is not true in all cases. As you scan through the list, you may notice that most of these service names are familiar application layer protocols.
Some protocols, such as FTP, use two ports, and Kerberos operates on either TCP or UDP (which typically means that one port functions as a backup port).
ftp-data 20/tcp #FTP, data ftp 21/tcp #FTP. control telnet 23/tcp domain 53/tcp #Domain Name Server domain 53/udp #Domain Name Server bootps 67/udp dhcps #Bootstrap Protocol Server bootpc 68/udp dhcpc #Bootstrap Protocol Client tftp 69/udp #Trivial File Transfer http 80/tcp www www-http #World Wide Web kerberos 88/tcp krb5 kerberos-sec #Kerberos kerberos 88/udp krb5 kerberos-sec #Kerberos ntp 123/udp #Network Time Protocol netbios-ns 137/tcp nbname #NETBIOS Name Service netbios-ns 137/udp nbname #NETBIOS Name Service netbios-dgm 138/udp nbdatagram #NETBIOS Datagram Service netbios-ssn 139/tcp nbsession #NETBIOS Session Service