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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 163.176.132.18
Destination IP Address 163.176.4.27
Source Port 2541
Destination Port 80
Protocol TCP
ACK Bit Yes
DATA User Data
TCP Port Service
20/21 FTP
23 Telnet
25 SMTP
80 WWW
144 News
UDP Port Service
161 SNMP