3Com corebuilder 3500 Implementation Manual page 487

For the source or destination IP address mask, you specify how many
parts of the IP address you want to match. Place a 255 in each portion of
the mask that you want the software to recognize; place a 0 in any
portion of the mask that you want the software to ignore.
The following examples show different ways of specifying IP addresses
and IP address masks:
An IP address of 192.101.20.254 with a mask of 255.255.255.255
requests an exact match for the host IP address 192.101.20.254.
An IP address of 192.101.20.0 with a mask of 255.255.255.0 requests
a match for any node on the subnet 192.101.20.0.
An IP address of 192.101.20.40 with a mask of 255.255.0.0 requests
a match for any node on the 192.101.0.0 network.
A destination IP address of 0.0.0.254 (or 192.101.20. 254) with a
mask of 0.0.0.255 requests a match on any node that ends in 254.
A mask of 0.0.0.0 is a wildcard match.
Specifying Ports and Port Ranges
Many common applications are associated with well-known port
numbers. For example, FTP (which uses TCP) uses ports 20 and 21, Telnet
(which also uses TCP) uses port 23, and SNMP (which uses UDP) uses port
161. You can consult the services database file typically associated with
TCP/IP hosts for a list of the well-known applications (services) and port
numbers. For other applications, you may have to determine the
appropriate port number.
When you specify the start and end range of a TCP or UDP port, specify
as small as range as possible, such as 1 port (for example, port 2049 as
both the start range and the end range). Also if possible, apply this small
range to both source and destination port ranges. If the classifier applies
to a wide range of TCP or UDP ports, you increase the amount of
classified traffic on the system and consume valuable QoS resources.
To define flow classifiers and their associated controls for specific
scenarios, see "Examples of Classifiers and Controls" later in this chapter.
QoS Classifiers 487