How An Ace Uses A Mask To Screen Packets For Matches; What Is The Difference Between Network (Or Subnet) Masks And The Masks Used With Acls - HP 3500yl Series Access Security Manual

Note
RACLs do filter routed or switched IPv4 traffic having an SA or DA on the
switch itself.
How an ACE Uses a Mask To Screen Packets for
Matches
When the switch applies an ACL to IPv4 traffic, each ACE in the ACL uses an
IPv4 address and ACL mask to enforce a selection policy on the packets being
screened. That is, the mask determines the range of IPv4 addresses (SA only
or SA/DA) that constitute a match between the policy and a packet being
screened.
What Is the Difference Between Network (or Subnet)
Masks and the Masks Used with ACLs?
In common IPv4 addressing, a network (or subnet) mask defines which part
of the address to use for the network number and which part to use for the
hosts on the network. For example:
Address Mask
10.38.252.195 255.255.255.0 first three octets
10.38.252.195 255.255.248.0 first two octets and the left-
Thus, the bits set to 1 in a network mask define the part of an IPv4 address to
use for the network number, and the bits set to 0 in the mask define the part
of the address to use for the host number.
In an ACL, IPv4 addresses and masks provide criteria for determining whether
to deny or permit a packet, or to pass it to the next ACE in the list. If there is
a match, the configured deny or permit action occurs. If there is not a match,
the packet is compared with the next ACE in the ACL. Thus, where a standard
network mask defines how to identify the network and host numbers in an
IPv4 address, the mask used with ACEs defines which bits in a packet's SA or
DA must match the corresponding bits in the SA or DA listed in an ACE, and
which bits can be wildcards.
Network Address
most five bits of the third octet
IPv4 Access Control Lists (ACLs)
Planning an ACL Application
Host Address
The fourth octet.
The right most three bits of the
third octet and all bits in the
fourth octet.
10-35