What is the highest port number one can use?
This question is related to
tcp
network-protocols
The largest port number is an unsigned short 2^16-1: 65535
A registered port is one assigned by the Internet Corporation for Assigned Names and Numbers (ICANN) to a certain use. Each registered port is in the range 1024–49151.
Since 21 March 2001 the registry agency is ICANN; before that time it was IANA.
Ports with numbers lower than those of the registered ports are called well known ports; port with numbers greater than those of the registered ports are called dynamic and/or private ports.
It depends on which range you're talking about, but the dynamic range goes up to 65535 or 2^16-1 (16 bits).
http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
According to RFC 793, the port is a 16 bit unsigned int.
This means the range is 0 - 65535.
However, within that range, ports 0 - 1023 are generally reserved for specific purposes. I say generally because, apart from port 0, there is usually no enforcement of the 0-1023 reservation. TCP/UDP implementations usually don't enforce reservations apart from 0. You can, if you want to, run up a web server's TLS port on port 80, or 25, or 65535 instead of the standard 443. Likewise, even tho it is the standard that SMTP servers listen on port 25, you can run it on 80, 443, or others.
Most implementations reserve 0 for a specific purpose - random port assignment. So in most implementations, saying "listen on port 0" actually means "I don't care what port I use, just give me some random unassigned port to listen on".
So any limitation on using a port in the 0-65535 range, including 0, ephemeral reservation range etc, is implementation (i.e. OS/driver) specific, however all, including 0, are valid ports in the RFC 793.
As I understand it, you should only use up to 49151, as from 49152 up to 65535 are reserved for Ephemeral ports
Valid numbers for ports are: 0 to 2^16-1 = 0 to 65535
That is because a port number is 16 bit length.
However ports are divided into:
Well-known ports: 0 to 1023 (used for system services e.g. HTTP, FTP, SSH, DHCP ...)
Registered/user ports: 1024 to 49151 (you can use it for your server, but be careful some famous applications: like Microsoft SQL Server database management system (MSSQL) server or Apache Derby Network Server are already taking from this range i.e. it is not recommended to assign the port of MSSQL to your server otherwise if MSSQL is running then your server most probably will not run because of port conflict )
Dynamic/private ports: 49152 to 65535. (not used for the servers rather the clients e.g. in NATing service)
In programming you can use any numbers 0 to 65535 for your server, however you should stick to the ranges mentioned above, otherwise some system services or some applications will not run because of port conflict.
Check the list of most ports here: https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
It should be 65535.
Just a followup to smashery's answer. The ephemeral port range (on Linux at least, and I suspect other Unices as well) is not a fixed. This can be controlled by writing to
/proc/sys/net/ipv4/ip_local_port_range
The only restriction (as far as IANA is concerned) is that ports below 1024 are designated to be well-known ports. Ports above that are free for use. Often you'll find that ports below 1024 are restricted to superuser access, I believe for this very reason.
Source: Stackoverflow.com