Standard Ports And Random Ports - Watchguard Firebox X20E Reference Manual

IPIP (IP-within-IP)
An encapsulation protocol used to build virtual networks over the Internet.
GGP (Gateway-Gateway Protocol)
A routing protocol used between autonomous systems.
GRE
A protocol used for PPTP.
ESP
An encryption protocol used for IPSec.

Standard ports and random ports

UDP and TCP encapsulate information contained within the application layer. The
appropriate application processes are designated by source and destination port
numbers. These port numbers, along with the source and destination IP addresses,
specify a unique connection on the Internet.
For example, it is reasonable to have two telnet sessions from one host to another.
However, since telnet uses a well-known service number of 23, something must
distinguish these two connections. The other port in these cases will be a port that is
typically greater than 1023. This alternative port designation is dynamically allocated
by the operating system on the client side.
Random ports can cause a great amount of trouble if they happen to match a well-
known service on a port above 1023. If some client machine assigns a random port of
2049, the connection may mysteriously fail. Similar problems can occur with the X
Window and Archie services.
In practice, most operating systems cycle port numbers between 1024 and a number
somewhere in the range of 2100, depending on how many TCP connections are
currently open and whether a recently closed connection used a similar port number.
This makes the above problem rare.
Reference Guide
Standard ports and random ports
7