Netopia Firmware 4000-Series - Manual (Page 261)

Security 10-37
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