Netopia 3346N-ENT User Guide (en)

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Firewall Tutorial
General firewall terms
Filter rule: A filter set is comprised of individual filter rules.
Filter set: A grouping of individual filter rules.
Firewall: A component or set of components that restrict access between a protected network and the Internet, 
or between two networks.
Host: A workstation on the network.
Packet: Unit of communication on the Internet.
Packet filter: Packet filters allow or deny packets based on source or destination IP addresses, TCP or UDP 
por ts, or the TCP ACK bit.
Port: A number that defines a par ticular type of ser vice.
Basic IP packet components
All IP packets contain the same basic header information, as follows:
This header information is what the packet filter uses to make filtering decisions. It is impor tant to note that a 
packet filter does not look into the IP data stream (the User Data from above) to make filtering decisions.
Basic protocol types
TCP: Transmission Control Protocol. TCP provides reliable packet deliver y and has a retransmission 
mechanism (so packets are not lost). RFC 793 is the specification for TCP.
UDP: User Datagram Protocol. Unlike TCP, UDP does not guarantee reliable, sequenced packet deliver y. 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.
Source IP Address
163.176.132.18
Destination IP Address
163.176.4.27
Source Por t
2541
Destination Por t
80
Protocol
TCP
ACK Bit
Yes
DATA
User Data