Port numbers
IP addresses, alone,
are not enough as they only address hosts,
and not individual operating system processes on those hosts.
From the perspective of any transport protocol,
such as TCP (next),
each arriving frame is further identified by a 16-bit positive
port number
that identifies the 'software end-point' to receive the payload.
One role of TCP is to demultiplexed each arriving
segment to its corresponding communication end-point,
using a port as an index.
Port numbers below 1024 are described as reserved ports,
and on operating systems with distinct users and privilege levels,
elevated privilege ('root' or 'administrator' access)
is required to create a 'software end-point' bound to such ports.
The file /etc/services on Linux and macOS,
or C:\Windows\System32\drivers\etc on Windows,
lists ports commonly used (worldwide),
and ports in use for dedicated/local applications:
// Ports below 1024 are reserved
echo 7/tcp
ftp 21/tcp # File transfer protocol
ssh 22/tcp # SSH Remote Login Protocol
telnet 23/tcp # Telnet
smtp 25/tcp # Simple mail transfer protocol
finger 79/tcp
http 80/tcp # Hypertext transfer protocol
pop3 110/tcp # POP version 3
sunrpc 111/tcp # RPC 4.0 portmapper TCP
nntp 119/tcp # Network news transfer protocol
https 443/tcp # Secured hypertext transfer protocol
exec 512/tcp # Remote execution
login 513/tcp
// Ports above 1023 are not reserved, but may be pre-assigned:
ms-sql-s 1433/tcp # Microsoft-SQL-Server
license 1702/tcp # matlab/FLEXlm licence manager
nfs 2049/udp # The network file system (NFS)
x11 6000/tcp # the X Window System
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CITS3002 Computer Networks, Lecture 8, Transport layer protocols and APIs, p2, 24th April 2024.
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CITS3002