Computer Networks - Lecture 8, p2

Computer Science & Software Engineering
Computer Networks (CITS3230) - Lecture 8

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Port numbers

IP addresses, alone, are not enough as they only address hosts, and not individual operating system processes.

From the perspective of any transport protocol, such as TCP, 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 end-point.

Port numbers below 1024 are described as reserved ports, and on operating systems with distinct users and priviledge levels, elevated privilege is required to create a 'software end-point' bound to such ports.

Under Unix this typically requires the process to have root priviledge.

The Unix/Linux file /etc/services lists common ports:


// 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

Computer Networks (CITS3230), Lecture 8, p2, 5th May 2008.