Implementing
VLSM
Medium-Sized Routed Network Construction
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ICND1 v1.01-1
Subnetting Review
To identify subnets, you will borrow bits from the host
ID portion of the IP address:
The number of subnets available depends on the number of bits
borrowed.
The available number of subnets = 2s, I which s is the number
of bits borrowed.
The number of hosts per subnet available depends upon the
number of host ID bits not borrowed.
The available number of hosts per subnet = 2h -2, in which h is
the number of host bits not borrowed.
One address is reserved as the network address.
One address is reserved as the broadcast address.
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ICND1 v1.01-2
Possible Subnets and Hosts for a
Class C Network
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ICND1 v1.01-3
Possible Subnets and Hosts for a
Class B Network
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ICND1 v1.01-4
Possible Subnets and Hosts for a
Class A Network
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ICND1 v1.01-5
Subnetting Review Exercise
Subnet a network with a private network address of
172.16.0.0./16 so that it provides 100 subnets and
maximizes the number of host addresses for each
subnet.
How many bits will need to be borrowed?
What is the new subnet mask?
What are the first four subnets?
What are the range of host addresses for the four subnets?
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ICND1 v1.01-6
What Is a Variable-Length Subnet Mask?
Subnet 172.16.14.0/24 is divided into smaller subnets.
Subnet with one mask (/27).
Then further subnet one of the unused /27 subnets into multiple /30 subnets.
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ICND1 v1.01-7
A Working VLSM Example
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ICND1 v1.01-8
A Working VLSM Example (Cont.)
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ICND1 v1.01-9
A Working VLSM Example (Cont.)
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ICND1 v1.01-10
A Working VLSM Example (Cont.)
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ICND1 v1.01-11
Understanding Route Summarization
Routing protocols can summarize addresses
of several networks into one address.
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ICND1 v1.01-12
Classful Routing Overview
Classful routing protocols do not include the subnet mask with the
network in the routing advertisement.
Within the same network, consistency of the subnet masks is
assumed, one subnet mask for the entire network.
Summary routes are exchanged between foreign networks.
Examples of classful routing protocols include:
RIPv1
IGRP
Note: Classful routing protocols are legacy routing protocols
typically used to address compatibility issues. RIP version 1 and
Interior Gateway Routing Protocol (IGRP) are introduced to
provide examples.
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ICND1 v1.01-13
Classless Routing Overview
Classless routing protocols include the subnet mask with the
network in the advertisement.
Classless routing protocols support VLSM; one network can have
multiple masks.
Summary routes must be manually controlled within the network.
Examples of classless routing protocols include:
RIPv2
EIGRP
OSPF
RIPv2 and EIGRP act classful by default, and summary routes are
exchanged between foreign networks.
The no auto-summary command forces these protocols to
behave as if they are classless.
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ICND1 v1.01-14
Summarizing Within an Octet
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ICND1 v1.01-15
Summarizing Addresses in a
VLSM-Designed Network
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ICND1 v1.01-16
Route Summarization Operation in
Cisco Routers
192.16.5.33
192.16.5.32
192.16.5.0
192.16.0.0
0.0.0.0
/32
/27
/24
/16
/0
Host
Subnet
Network
Block of Networks
Default
Supports host-specific routes, blocks of networks,
and default routes
Routers use longest prefix match
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ICND1 v1.01-17
Summarizing Routes in a
Discontiguous Network
Classful RIPv1 and IGRP do not advertise subnets,
and therefore cannot support discontiguous subnets.
Classless OSPF, EIGRP, and RIPv2 can advertise subnets,
and therefore can support discontiguous subnets.
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ICND1 v1.01-18
Summary
Subnetting lets you efficiently allocate addresses by taking one
large broadcast domain and breaking it up into smaller more
manageable broadcast domains.
VLSMs let you more efficiently allocate IP addresses by adding
multiple layers of the addressing hierarchy.
The benefits of route summarization include smaller routing tables
and the ability to isolate topology changes.
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ICND1 v1.01-19
