Motorola Netopia 3300 Guia do Utilizador descarregar pdf (Página 110)

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Here is what this rule looks like when

implemented as a ﬁlter in your Gate-

way:

To understand this particular ﬁlter, look

at the parts of a ﬁlter.

Parts of a ﬁlter

A ﬁlter consists of criteria based on

packet attributes. A typical ﬁlter can

match a packet on any one of the fol-

lowing attributes:

•The source IP address and subnet

mask (where the packet was sent

from)

•The destination IP address and sub-

net mask (where the packet is going)

• The TOS bit setting of the packet. Certain types of IP packets, such as voice or multime-

dia packets, are sensitive to delays introduced by the network. A delay-sensitive packet

is identiﬁed by a special low-latency setting called the TOS bit. It is important for such

packets to be received rapidly or the quality of service degrades.

• The type of higher-layer Internet protocol the packet is carrying, such as TCP or UDP

Port numbers

A ﬁlter can also match a packet’s port number attributes, but only if the ﬁlter’s protocol

type is set to TCP or UDP, since only those protocols use port numbers. The ﬁlter can be

conﬁgured to match the following:

• The source port number (the port on the sending host that originated the packet)

• The destination port number (the port on the receiving host that the packet is destined

for)

By matching on a port number, a ﬁlter can be applied to selected TCP or UDP services,

such as Telnet, FTP, and World Wide Web. The following tables show a few common ser-

vices and their associated port numbers:

1 2 ... 105 106 107 108 109 110 111 112 113 114 115 ... 278 279

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Motorola Netopia 3300 Guia do Utilizador Página 110