Security 8-75

Example IP filters

Example 1

Write a filter rule that blocks the class C subnet represented by 200.1.1.0/25 from accessing the net.

Filter Rule:

200.1.1.0

(Source IP Network Address)

 

 

 

 

255.255.255.128

(Source IP Mask)

 

 

 

 

Forward = No

(What happens on match)

 

 

 

Incoming packet has the source address of 200.1.1.28

To determine if the packet will match on the filter, perform a Boolean AND on the source IP address and the filter’s source IP mask:

IP Address

Binary Representation of

 

 

the last byte of the IP

 

 

address

 

 

 

 

200.1.1.28

00011100

(Source address in incoming IP packet)

 

 

 

AND

 

 

 

 

 

255.255.255.128

10000000

(Perform the logical AND)

 

 

 

 

00000000

(Logical AND result)

 

 

 

This incoming IP packet has a source IP address that matches the network address in the Source IP Address field (whose last byte is binary 00000000) in the Netopia D-Series. This will not forward this packet.

Example 2

 

 

Filter Rule:

 

200.1.1.0

 

(Source IP Network Address)

 

 

 

 

 

 

 

 

 

 

 

 

 

255.255.255.128

 

(Source IP Mask)

 

 

 

 

 

 

 

 

 

 

 

 

 

Forward = No

 

(What happens on match)

 

 

 

 

 

 

 

 

 

Incoming packet has the source address of 200.1.1.184.

 

 

 

 

 

 

 

 

 

 

IP Address

Binary Representation

 

 

 

 

 

 

 

 

200.1.1.184

10111000

(Source address in incoming IP packet)

 

 

 

 

 

 

 

 

AND

 

 

 

 

 

 

 

 

 

 

 

 

 

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Netopia D3232 IDSL, D7171 SDSL, D7100 SDSL, D3100-I IDSL manual Example IP filters

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