Defining a NAT pool: When creating a pool of global addresses on Cisco IOS, what keyword can be used instead of the <em>netmask</em> parameter to specify mask length?

Difficulty: Easy

Correct Answer: prefix-length

Explanation:


Introduction / Context:
Dynamic NAT typically requires defining a pool of global (translated) addresses. Cisco IOS lets you specify the pool's subnet mask either as a dotted-decimal netmask or by an alternative parameter. Knowing both forms is useful for speed and accuracy.



Given Data / Assumptions:

  • You are configuring ip nat pool for dynamic or policy NAT.
  • You want to avoid typing a dotted-decimal mask.
  • Standard IOS NAT syntax applies.


Concept / Approach:

The ip nat pool command supports either netmask a.b.c.d or prefix-length n, where n is the CIDR length (for example, 24 for 255.255.255.0). There is no slash notation in the command itself; you spell out prefix-length.



Step-by-Step Solution:

Enter global configuration.Define pool: ip nat pool NAME 203.0.113.10 203.0.113.50 prefix-length 27Reference the pool from a NAT rule and apply inside/outside on interfaces.


Verification / Alternative check:

Use show running-config to confirm that the pool reflects the intended prefix length. Test with live traffic or show ip nat translations.



Why Other Options Are Wrong:

Slash notation (A) is not literal CLI syntax for this command.

“No mask” (C) would be invalid; a mask is required.

“Block-size” (D) is unrelated to ip nat pool mask notation.



Common Pitfalls:

Typing area-style slash prefixes as in OSPF; mixing up pool prefix with inside interface subnet; forgetting to allow traffic with the correct ACL or route-map.



Final Answer:

prefix-length

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