Netopia R2020 - Manual (Page 233)

Security 14-29
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.
Filter Rule: A filter set is comprised of individual filter rules.
Filter Set: A grouping of individual filter rules.
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 datastream (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 re transmit the data. RFC 768 is the specification for UDP.
And there are many more por ts defined in the Assigned Addresses RFC.
Example TCP/UDP Ports
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
TCP Port Service UDP Port Service
20/21 FTP 161 SNMP
23 Telnet 69 TFTP
25 SMTP 387 AURP