HP ProCurve 6120G/XG Manual page 354

IPv4 Access Control Lists (ACLs)
Traffic Management and Improved Network Performance
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 9-3, below.
Table 9-3.
Location of Octet
SA in ACE
Mask for SA
Corresponding Octet of a Packet's
SA
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 9-7 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.
9-30
Example of How the Mask Defines a Match
128
0
0
0
Bit Position in the Octet
64 32 16 8
0 0 1 1
0 0 0 0
0 0 1 1 0/1
4 2 1
1 1 1
1 1 1
0/1 0/1