Motorola 3342 사용자 설명서

UDP: User Datagram Protocol. Unlike TCP, UDP does not guarantee reliable, sequenced packet delivery. If 
data does not reach its destination, UDP does not retransmit the data. RFC 768 is the specification for 
UDP.

There are many more por ts defined in the Assigned Addresses RFC. The table that follows shows some of 
these por t assignments.

There are two basic rules to firewall design:

•

“What is not explicitly allowed is denied.”

and

•

“What is not explicitly denied is allowed.”

The first rule is far more secure, and is the best approach to firewall design. It is far easier (and more 
secure) to allow in or out only cer tain ser vices and deny anything else. If the other rule is used, you would 
have to figure out ever ything that you want to disallow, now and in the future. 

Firewall design is a test of logic, and filter rule ordering is critical. If a packet is for warded through a series 
of filter rules and then the packet matches a rule, the appropriate action is taken. The packet will not for-
ward through the remainder of the filter rules.

For example, if you had the following filter set...

Allow WWW access;

Allow FTP access;

Allow SMTP access;

Deny all other packets.

and a packet goes through these rules destined for FTP, the packet would for ward through the first rule 
(WWW), go through the second rule (FTP), and match this rule; the packet is allowed through.

If you had this filter set for example....

Allow WWW access;

20/21

FTP

161

SNMP

23

Telnet

69

TFTP

25

SMTP

80

WWW

144

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