Netopia R3100 manual Binary Representation, Logical ANDing, Implied Rules

Models: R3100

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15-32 User’s Reference Guide

and a packet goes through these rules destined for FTP, the packet would pass through the first filter rule (WWW), match the second rule (FTP) and the packet is allowed through. Even though the next rule is to deny all FTP traffic, the FTP packet will never make it to this rule.

Binary Representation

It is easiest when doing filtering to convert the IP address and mask in question to binary. This will allow you to perform the logical AND to determine if a packet matches a filter rule.

Logical ANDing

When a packet is compared (in most cases) a logical AND is performed. First the IP addresses and subnet masks are converted to binary and then ANDed together. The rules for logical ANDing are as follows:

0 AND 0 = 0

 

 

0 AND 1 = 0

 

 

1 AND 0 = 0

 

 

1 AND 1 = 1

 

 

For example:

 

 

Filter rule:

 

 

Deny

 

 

IP: 163.176.1.15

BINARY:

10100011.10110000.00000001.00001111

Mask: 255.255.255.255

BINARY: 11111111.11111111.11111111.11111111

Incoming Packet:

 

 

IP 163.176.1.15

BINARY:

10100011.10110000.00000001.00001111

AND the incoming packet and subnet mask together, the result is:

10100011.10110000.00000001.00001111

which matches the IP address in the filter rule and the packet is denied.

Implied Rules

With a given set of filter rules, there is an Implied rule which 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:

Implied

Meaning

 

 

Y+Y+Y=N

If all filter rules are YES, the implied is NO.

 

 

N+N+N=Y

If all filter rules are NO, the implied is YES.

 

 

Y+N+Y=N

If a mix of YES and NO filters, the implied is NO.

 

 

Page 210
Image 210
Netopia R3100 manual Binary Representation, Logical ANDing, Implied Rules