HP 6400cl, Management Manual, Page: 369 / 664

Access Control Lists (ACLs) for the Series 5300xl Switches
Planning an ACL Application
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 9-2, below.
Table 9-2. 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 9-5 to filter
inbound packets on VLAN 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
18.28.252.117 0.0.0.0
exit
Inbound Packet “A” On VLAN 20
– Destination Address: 18.35.248.184
– Source Address: 18.28.252.117
Inbound Packet “B” On VLAN 20
– Destination Address: 18.35.248.184
– Source Address: 18.28.252.120
This ACL (a standard ACL named “Fileserver”) includes an ACE (Access Control Entry)
that permits matches only with the packets received from IP address 18.28.252.117
(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 VLAN permits packet “A”
because it s source IP address
matches the source address in the
ACE.
The VLAN denies packet “B”
because its source IP address does
not match the source address in the
ACE.
ACE
Figure 9-5. Example of an ACL with an Access Control Entry (ACE) that Allows Only One Source IP Address
9-23