Access Control Implicit Deny; Ip Addresses Used For Access Lists When You Use Nat - Cisco PIX 500 Series Configuration Manual

Chapter 16 Identifying Traffic with Access Lists
You can disable an ACE by specifying the keyword inactive in the access-list command.

Access Control Implicit Deny

Access lists have an implicit deny at the end of the list, so unless you explicitly permit it, traffic cannot
pass. For example, if you want to allow all users to access a network through the security appliance
except for particular addresses, then you need to deny the particular addresses and then permit all others.
For EtherType access lists, the implicit deny at the end of the access list does not affect IP traffic or
ARPs; for example, if you allow EtherType 8037, the implicit deny at the end of the access list does not
now block any IP traffic that you previously allowed with an extended access list (or implicitly allowed
from a high security interface to a low security interface). However, if you explicitly deny all traffic with
an EtherType ACE, then IP and ARP traffic is denied.

IP Addresses Used for Access Lists When You Use NAT

When you use NAT, the IP addresses you specify for an access list depend on the interface to which the
access list is attached; you need to use addresses that are valid on the network connected to the interface.
This guideline applies for both inbound and outbound access lists: the direction does not determine the
address used, only the interface does.
For example, you want to apply an access list to the inbound direction of the inside interface. You
configure the security appliance to perform NAT on the inside source addresses when they access outside
addresses. Because the access list is applied to the inside interface, the source addresses are the original
untranslated addresses. Because the outside addresses are not translated, the destination address used in
the access list is the real address (see
Figure 16-1
Permit from
10.1.1.0/24
See the following commands for this example:
hostname(config)# access-list INSIDE extended permit ip 10.1.1.0 255.255.255.0 host
209.165.200.225
OL-12172-03
IP Addresses in Access Lists: NAT Used for Source Addresses
209.165.200.225
Outside
Inside
Inbound ACL
10.1.1.0/24 to 209.165.200.225
10.1.1.0/24 209.165.201.4:port
PAT
Figure 16-1).
Cisco Security Appliance Command Line Configuration Guide
Access List Overview
16-3