Access Control Lists (ACLs) for the Series 3400cl and Series 6400cl Switches
Planning an ACL Application on a Series 3400cl or Series 6400cl Switch
Example of How the Mask Bit Settings Define a Match . Assume an
ACE where the second octet of the mask for an SA is 7 (the rightmost three
bits are “on”, or “1”) and the second octet of the corresponding SA in the ACE
is 31 (the rightmost five bits). In this case, a match occurs when the second
octet of the SA in a packet being filtered has a value in the range of 24 to 31.
Refer to table
10-4, below.
Table 10-4. Example of How the Mask Defines a Match
Location of Octet Bit Position in the Octet
128 64 32 16 8 4 2 1
SA in ACE
Mask for SA
Corresponding Octet of a Packet’s
SA
0
0
0
0
0
0
0
0
0
1
0
1
1
0
1
1
1
0/1
1
1
0/1
1
1
0/1
The shaded area indicates bits in the packet that must exactly match the bits in the source
IP in the ACE. Wherever the mask bits are ones (wildcards), the IP bits in the packet can be
any value, and where the mask bits are zeros, the IP bits in the packet must be the same as
the IP bits in the ACE. Note: This example covers only one octet of an IP address. An actual
ACE applies this method to all four octets of an IP address.
Example of Allowing Only One IP Address (“Host” Option). Sup-
pose, for example, that you have configured the ACL in figure 10-8 to filter
inbound packets on port 20. Because the mask is all zeros, the ACE policy
dictates that a match occurs only when the source IP address on such packets
is identical to the IP address configured in the ACE.
ip access-list standard Fileserver
permit 10.28.252.117 0.0.0.0
exit
Inbound Packet “A” On Port 20
– Destination Address: 10.35.248.184
– Source Address: 10.28.252.117
Inbound Packet “B” On Port 20
– Destination Address: 10.35.248.184
– Source Address: 10.28.252.120
This ACL (a standard ACL named “Fileserver”) includes an ACE (Access Control Entry)
that permits matches only with the packets received from IP address 10.28.252.117
(the SA). Packets from any other source do not match and are denied.
Source IP Address (SA)
Mask (All zeros mean that a match occurs
only when an IP packet’s source address is
identical to the source address configured
in the ACE.
The port permits packet “A” because
its source IP address matches the
source address in the ACE.
The port denies (drops) packet “B”
because its source IP address does
not match the source address in the
ACE.
ACE
Figure 10-8. Example of an ACL with an Access Control Entry (ACE) that Allows Only One Source IP Address
10-33
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