CCNA 3 Labs and Study Guide - BINARYBB.INFO – @jagalbraith

Concept Questions
List at least three reasons why you should use VLSM when designing your addressing scheme.
■ More efficient use of IP addresses
■ Greater capability to use route summarization
■ Isolation of topology changes from other routers
Why is VLSM described as “subnetting a subnet”?
From the instructor version of the curriculum:
VLSM is often referred to as “subnetting a subnet” because any network address space—whether a classful
address like 192.168.1.0/24 or a classless address like 192.168.1.32/27—can be further subnetted to
provide another level of logical addressing.
Why was VLSM not used in CCNA 1 and CCNA 2?
From the instructor version of the curriculum:
There are two main reasons why VLSM is not used in CCNA 1 and CCNA 2:
■ Historically, subnetting has proved to be one of the more challenging skills students must master during
the first two CCNA courses. Adding the concept of VLSM to this already difficult task is unnecessary,
namely because….
■ CCNA 2 only deals with classful routing. Students do not yet implement classless addressing schemes
in their network designs.
What is the difference between CIDR and supernetting or router summarization?
From the instructor version of the curriculum:
Classless Interdomain Routing (CIDR) is the mechanism that allows advertising of both supernets and subnets
outside of the normal bounds of a classful network number. Supernetting is a representation that
allows masks that are shorter than the natural masks, therefore creating supernets.
From Switching Basics and Intermediate Routing CCNA 3 Companion Guide, by Wayne Lewis (Cisco
Networking Academy Program):
Although there is no consensus, the term route summarization often applies to summarizing within a classful
boundary; on the other hand, CIDR almost always refers to combining several classful networks. With
both CIDR and route summarization, the point is to optimize routing. To illustrate the difference between
route summarization and CIDR, a network engineer may define a summary route on a Cisco router for a
company’s network, but this has nothing to do with allocating a block of addresses to a customer.
List the two ways a router running a classful routing protocol can calculate the network portion of routes
received in routing updates.
From Switching Basics and Intermediate Routing CCNA 3 Companion Guide:
■ If the routing update information contains the same major network number as configured on the
receiving interface, the router applies the subnet mask that is configured on the receiving interface.
■ If the routing update information contains a different major network than the one configured on the
receiving interface, the router applies the default classful mask by IP address class.
Explain three ways a router can learn paths to destination networks.
From Switching Basics and Intermediate Routing CCNA 3 Companion Guide:
Chapter 1: Introduction to Classless Routing 31
■ Static routes are manually defined by the system administrator via an attached interface or the next
hop to a destination. These are useful for security and reducing routing traffic.