What is the largest TCP/IP network port number allowable for IPv4?

As I understand it, you should only use up to 49151, as from 49152 up to 65535 are reserved for Ephemeral ports

The port number is an unsigned 16-bit integer, so 65535.

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.

Wikipedia: Registered Ports