Now let's talk about using a wildcard address object in a Source Address or Destination Address of a Security rule. When you assign private IPv4 addresses to internal devices, you can use an IP addressing structure that assigns meaning to certain bits in the address. For example, the first three bits in the third octet of an IP address signify the device type. This structure helps you easily identify details about a device, such as device type or location, based on the IP address of the device. You can use this same IP addressing structure in Security rules for easier deployment. You create an address object that uses a wildcard address (IP address and wildcard mask separated by a slash, such as 10.1.2.3/0.127.248.0). A wildcard address can identify many source or destination addresses in a single Security rule, which is especially helpful for data centers serving many devices. You won’t have to manage an unnecessarily large number of address objects to cover all the matching IP addresses or use less restrictive Security rules than you need due to IP address capacity constraints.

For example, suppose you use the IPv4 addressing scheme shown in the following figure where the first octet represents your organization (bits 00001010 are fixed). In the second octet, the first four bits designate the country where the network device is located (1000 indicates the U.S.) and the last four bits indicate the region (0100 indicates the northeast). In the third octet, the first four bits are zeros and the last four bits indicate device type (0001 indicates cash register and 0011 indicates printer). The last octet indicates the ID number of the networking device.

Based on that structure, the IP address of cash register number 156 in the northeastern U.S. would be 10.132.1.156:

You can use an address object of type

IP Wildcard Mask

to support such an addressing structure in a Security rule. You apply a wildcard mask to an IPv4 source or destination address to specify which addresses are subject to the rule. In a Palo Alto Networks wildcard mask, a zero bit indicates that the bit being compared must match the bit in the IP address that is covered by the zero. A one bit in the mask is a wildcard or “ignore” bit, meaning the bit being compared need not match the bit in the IP address. For example, the following snippets of an IP address and wildcard mask illustrate how they yield four matches:

In the example, cash registers have an IPv4 address with the third octet 00000001 and printers have an IPv4 address with the third octet 00000011. Suppose you want to apply a Security rule to all cash registers and printers having any ID number from 0 to 255. To get that result, you need a wildcard mask; the third octet of the wildcard mask must be 2 and the device ID (the fourth octet) must be 255. The address object to specify all cash registers and printers in the northeastern U.S. would use wildcard address 10.132.1.2/0.0.2.255:

Thus, a single Security rule that uses an address object with wildcard address 10.132.1.2/0.0.2.255 as the destination address matches the addresses of 512 devices (256 cash registers + 256 printers), which is an efficient way to apply a rule to many devices. The wildcard mask must begin with at least one zero (0), such as 0.0.2.255.

Consider the following when you use an address object of type

IP Wildcard Mask

in a Security rule:

The behavior in the preceding bullet is the default behavior. However, there are use cases where you want to have broad rules that allow some sources access to generic applications (such as Ping, Traceroute, and web-browsing), but have narrower rules that allow a subset of these sources access to different applications (such as SSH, SCP) in addition to the generic applications. In earlier releases, such a deployment did not work because only the match to the rule with the longest prefix in the wildcard mask was processed and other rules were not considered.

Beginning with

PAN-OS 10.2.1

, you can enable

Wildcard Top Down Match Mode

so that if a packet with an IP address matches prefixes in Security rules that have overlapping wildcard masks, the first fully matching rule in top-down order is chosen (instead of choosing the matching rule with the longest prefix in a wildcard mask). A packet is found to match the prefix in rules that use overlapping wildcard masks; then those rules that fully match all address bits based on masking are chosen, keeping in mind the ones in the mask indicate wildcard or “ignore” bits. Then other rule criteria, such as application and zones, are examined. During the other rule criteria examination, the first of those rules (in top-down order) that match the criteria is chosen. No other rules are evaluated.

The

Wildcard Top Down Match Mode

means that more than one rule has the potential to be enforced on different packets (not just the rule with the longest matching prefix). Place your more specific rules toward the top of the list. For example, you can allow a smaller range of matching addresses (a longer wildcard mask) to access certain applications, and also, in a subsequent rule allow a larger range of IP addresses (a shorter wildcard mask) to access a different (more generic) set of applications. You can enable

Wildcard Top Down Match Mode

by selecting

Device

Setup

Management

and editing the Security Policy Rulebase Settings.

The following example has

Wildcard Top Down Match Mode

enabled and three Security rules, each specifying a source IP address with wildcard mask address object, and the wildcard masks overlap:

In this example, Client A with source IP address 10.143.8.1 (0000 1010 1000 1111 0000 1000 0000 0001) fully matches Rule 1, Rule 2, and Rule 3; the first match is to Rule 1 (top-down order). Assuming other rule criteria match, the packet from Client A is subject to the Rule 1 action.

Client B with source IP address 10.160 2.1 (0000 1010 1010 0000 0000 0010 0000 0001) does not fully match the address in Rule 1 and does not match the prefix in Rule 2. Client B’s address fully matches Rule 3, which is the first matching rule in top-down order. Assuming other rule criteria match, the packet from Client B is subject to the Rule 3 action. Thus we see the benefit of

Wildcard Top Down Match Mode

, that both Rule 1 and Rule 3 can be in effect on different packets.