1 tcpmux TCP port service multiplexer transmission control protocol port service multiplexer
2 compressor management utility compressor management utility
3 compression network compression process compression process
5 rje remote job input remote job login
7 echo echo echo
9 discard discard discard discard
1 1 Systat active users online users
13 daytime daytime
17 daily reference
18 msp messaging protocol messaging protocol
19 character generator character generator
20 FTP- data file transfer [default data] file transfer protocol (default data port)
2 1 FTP file transfer [control] file transfer protocol (control)
22 ssh remote login protocol ssh remote login protocol
Terminal analog protocol
24 ? Any private mail system is reserved for personal mail systems.
25 smtp Simple Mail Transfer Simple Mail Transfer Protocol
27 nsw-fe NSW User System fe NSW User System Field Engineer
29msg-icpmsg icpmsg icp31msg-authmsg authentication message verification
33 dsp display support protocol
35 ? Any dedicated printer server is reserved for personal printer services.
37 time time
38 rap routing access protocol routing access protocol
39 rlp Resource Location Protocol Resource Location Protocol 4 1 Graphics
42 name server wins hostname server wins hostname service
43 nicname Who is the nickname and who is the service?
44 mpm-flags mpm FLAGS protocol MPM (message processing module)
45 MPM message processing module [recv] message processing module
46 MPM-SNMP[ default sending] message processing module (default sending port)
47 ni-ftp NI FTP NI FTP
48 auditd Digital Audit Daemon Digital Audio Background Service
49 TACACS Login Host Protocol (TACACS) TACACS Login Host Protocol
CK remote mail inspection protocol
5 1LA-maintimp logical address maintenance imp (Interface Information Processor) logical address maintenance
52 xns-time XNS time protocol Xerox network service system time protocol
53 domain name server domain name server
54 xns-ch Xerox Network Service System xns Clearing House
55 isi-gl ISI graphic language
Verifying xerox network service system with 56 xns-auth XNS authentication
57 ? Any private terminal access reserves personal terminal access.
……
Generally, ports 20 and 2 1 are both used for FTP protocol, 2 1 is used for transmitting control information, and 20 is used for transmitting data after the link is established.