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12-20 Administration Guide
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 important 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 delivery 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 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 ports defined in the Assigned Addresses RFC. The table that follows shows some of these
port assignments.
Example TCP/UDP Ports
Source IP Address
Destination IP Address
Source Port
Destination Port
Protocol
ACK Bit
DATA
TCP Port
20/21
23
25
80
144
UDP Port
161
163.176.132.18
163.176.4.27
2541
80
TCP
Yes
User Data
Service
FTP
Telnet
SMTP
WWW
News
Service
SNMP
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