The /etc/services file as a small local database. A list of internet network services.
For each service this file specifies the service’s well-known port number and notes whether the service is available as a TCP or UDP service
$ cat /etc/services
#
# Network services, Internet style
#
tcpmux 1/tcp
echo 7/tcp
echo 7/udp
discard 9/tcp sink null
discard 9/udp sink null
systat 11/tcp users
daytime 13/tcp
daytime 13/udp
netstat 15/tcp
chargen 19/tcp ttytst source
chargen 19/udp ttytst source
ftp-data 20/tcp
ftp 21/tcp
ssh 22/tcp # Secure Shell
telnet 23/tcp
smtp 25/tcp mail
time 37/tcp timserver