Netopia D3232 User Reference Manual page 73

0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1
For example:
Filter rule:
Deny
IP: 163.176.1.15BINARY:
Mask: 255.255.255.255BINARY:11111111.11111111.11111111.11111111
Incoming Packet:
IP 163.176.1.15BINARY:
If you put the incoming packet and subnet mask together with AND, the result is:
which matches the IP address in the filter rule and the packet is denied.
With a given set of filter rules, there is an Implied rule that may or may not be shown to the user. The implied
rule tells the filter set what to do with a packet that does not match any of the filter rules. An example of implied
rules is as follows:
Y+Y+Y=N
N+N+N=Y
Y+N+Y=N
The TCP header contains one bit called the ACK bit (or TCP Ack bit). This ACK bit appears only with TCP, not
UDP. The ACK bit is part of the TCP mechanism that guaranteed the delivery of data. The ACK bit is set
whenever one side of a connection has received data from the other side. Only the first TCP packet will not have
the ACK bit set; once the TCP connection is in place, the remainder of the TCP packets with have the ACK bit
set.
The ACK bit is helpful for filter design and reduces the number of potential filter rules. A filter rule could be
created just allowing incoming TCP packets with the ACK bit set, since these packets had to be originated from
the local network.
10100011.10110000.00000001.00001111
10100011.10110000.00000001.00001111
10100011.10110000.00000001.00001111
Implied
If all filter rules are YES, the implied rule is NO.
If all filter rules are NO, the implied rule is YES.
If a mix of YES and NO filters, the implied rule is NO.
Meaning
Security 8-73