Watchguard Firebox X1000, Reference Manual

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Standard Ports and Random Ports
Reference Guide 9
IGMP (Internet Group Multicast Protocol)
A protocol primarily designed for hosts on multiaccess networks
to inform locally attached routers of their group membership
information.
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.