When defining a pool of global addresses for Cisco NAT, which keyword can be used in place of the netmask parameter to specify the prefix length of the subnet?

Introduction / Context:
When configuring a dynamic NAT pool on a Cisco router, you must specify both a range of global IP addresses and the subnet information for that range. Traditionally this is done with the netmask keyword followed by a dotted decimal subnet mask. Newer IOS versions also allow you to specify the subnet using prefix length notation, which can be more convenient and less error prone.

Given Data / Assumptions:

You are creating an ip nat pool statement on a Cisco router.

You know that the command syntax supports netmask to define the subnet.

You want to use an alternative form that uses a prefix length instead.

One of the options corresponds to the actual Cisco IOS keyword for this feature.

Concept / Approach:
Cisco IOS supports two main ways to describe subnet information in NAT pool commands. The first is netmask, where you give a dotted decimal mask such as 255.255.255.224. The second is prefix-length, where you specify a numeric prefix such as 27. For example, ip nat pool Corp 198.18.41.129 198.18.41.134 prefix-length 29 is equivalent to using netmask 255.255.255.248. The actual keyword in the command is prefix-length, not a slash character by itself.

Step-by-Step Solution:
Recall the generic syntax for NAT pool configuration: ip nat pool name start-ip end-ip netmask mask or ip nat pool name start-ip end-ip prefix-length value. Identify the keyword in this syntax that replaces netmask; it is prefix-length followed by a number such as 24 or 27. Note that the slash symbol often used in CIDR notation, such as slash 24, is not used directly in the command as a standalone keyword. Compare the provided options with this knowledge and see that prefix-length is the correct IOS keyword. Therefore, option b is the correct answer.

Verification / Alternative check:
If you enter ip nat pool Test 10.1.1.1 10.1.1.6 ? at the CLI, IOS will display available keywords, including netmask and prefix-length. Choosing prefix-length 29 will configure the pool to use a slash 29 subnet. This kind of interactive help confirms the exact spelling of the keyword and demonstrates that it can be used in place of netmask.

Why Other Options Are Wrong:
Option a mentions slash notation, which is a way humans often write prefix lengths, such as 10.1.1.0 slash 24, but the literal slash character is not used as a keyword in the NAT pool command.

Common Pitfalls:
Option c, no mask, sounds like disabling a mask but does not exist as an IOS keyword in this context.
Option d, block-size, might appeal to those thinking in terms of number of addresses, but Cisco NAT does not use this keyword in pool definitions.
Option e, subnet-length, is not a Cisco command keyword and is likely included only as a distractor.

Common Pitfalls:
A frequent mistake is inconsistency between the pool range and the subnet definition. Whether you use netmask or prefix-length, ensure that the specified subnet correctly covers all addresses in the range and aligns with the actual network design. Using show ip nat statistics after configuration can help you verify that the pool is recognised correctly.

Final Answer:
Instead of netmask, you can use the prefix-length keyword to specify the subnet prefix length in a NAT pool definition.