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
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