CCNA ICND2 640-816 CH 5x (new window)

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CCNA ICND2
640
-
816

Official Cert Guide

Third Edition

Chapter
5

Variable Length Subnet Masks

Objectives


Define Variable Length Subnet Masking
(VLSM)


Configure VLSM


Find VLSM Overlaps


Add a subnet to an existing VLSM design


Design a complete VLSM
subnetting

plan

What is VLSM?


VLSM uses more than one subnet mask for
subnets within
a single
classful

network.


It is
not

just using different lengths masks in
different parts of the internetwork.


VLSM allows the size of the subnet to more
closely match the number of devices


Because of this, IP addresses are not wasted,
and the address space is used more
effciently
.

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
1 VLSM in Network 10.0.0.0: Masks /24 and /30

VLSM

and Routing Protocols


To support VLSM the routing protocol
must send the mask when it advertises a
subnet.

Routing
Protocol

Classless?

Sends Mask
in Updates?

Supports

VLSM?

Manual Route
Summarization

RIP
-
1

No

No

No

No

IGRP

No

No

No

No

RIP
-
2

Yes

Yes

Yes

Yes

EIGRP

Yes

Yes

Yes

Yes

OSPF

Yes

Yes

Yes

Yes

Configuring VLSM


Configure your router to use RIP
-
2,
EIGRP, or OSPF

Note:

You must turn off auto
-
summarization in
RIP
-
2 and EIGRP using the
no auto
-
summary

command


Configuring interfaces with different
subnet masks and addresses in the same
classful

network enables VLSM.

Yosemite#configure

terminal

Yosemite(
config
)#interface Fa0/0

Yosemite(
config
-
if)#
ip

address 10.2.1.1
255.255.255.0

Yosemite(
config
-
if)#interface S0/1

Yosemite(
config
-
if)#
ip

address 10.1.4.1
255.255.255.252


When we configure addresses in 10.0.0.0 with a
/24 mask on Fa0/0 and /30 on S0/1, the IOS
detects that VLSM is being used.

Configuring VLSM (cont.)

Finding VLSM Overlaps

1)
Calculate
the subnet ID and subnet broadcast
address of each subnet, which
gives you
the
range of addresses in that subnet.

2)
List
the subnet IDs in numeric order (along with
their subnet
broadcast addresses
).

3)
Scan
the list top to bottom, comparing each
pair of adjacent entries, to
see if
their range of
addresses overlaps.

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
2 VLSM Design with Possible Overlap

Finding VLSM Overlaps (
cont
)

Subnet

Subnet Number

Broadcast Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.5.0

172.16.5.255

R1
-
R2 Serial

172.16.9.0

172.16.9.3

R1
-
R3 Serial

172.16.9.4

172.16.9.7

Steps 1 and 2 for the network in Figure 5
-
2

Finding VLSM Overlaps (
cont
)

Subnet

Subnet Number

Broadcast Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.5.0

172.16.5.255

R1
-
R2 Serial

172.16.9.0

172.16.9.3

R1
-
R3 Serial

172.16.9.4

172.16.9.7

As we go through step 3 we find…

Finding VLSM Overlaps (
cont
)

Subnet

Subnet Number

Broadcast Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.5.0

172.16.5.255

R1
-
R2 Serial

172.16.9.0

172.16.9.3

R1
-
R3 Serial

172.16.9.4

172.16.9.7

The highlighted subnets overlap!

Adding to an existing VLSM Design

1)
Pick the new subnet mask (if not given)

2)
Calculate all possible subnets using that mask

3)
List the existing subnet numbers with their
broadcast
addressess

4)
Identify overlapping subnets in the two lists

5)
Choose the new subnet from the remaining
subnets.

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
3 Internetwork to Which You Need to Add a /23 Subnet, Network 172.16.0.0

Finding VLSM Overlaps (
cont
)

Subnet

Subnet Number

Broadcast Address

First

172.16.0.0

172.16.1.255

Second

172.16.2.0

172.16.3.255

Third

172.16.4.0

172.16.5.255

Fourth

172.16.6.0

172.16.7.255

Fifth

172.16.8.0

172.16.9.255

Step 1: We are given /23

Step 2: List the first five subnets

Finding VLSM Overlaps (
cont
)

Subnet

Subnet Number

Broadcast Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.6.0

172.16.6.255

R1
-
R2 Serial

172.16.9.0

172.16.9.3

R1
-
R3 Serial

172.16.9.4

172.16.9.7

S
tep 3: List the existing subnets

Finding VLSM Overlaps (
cont
)

Subnet

Subnet
Number

Broadcast
Address

First

172.16.0.0

172.16.1.255

Second

172.16.2.0

172.16.3.255

Third

172.16.4.0

172.16.5.255

Fourth

172.16.6.0

172.16.7.255

Fifth

172.16.8.0

172.16.9.255

Step 4: Compare the two lists for overlaps

Subnet

Subnet
Number

Broadcast
Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.6.0

172.16.6.255

R1
-
R2
Serial

172.16.9.0

172.16.9.3

R1
-
R3
Serial

172.16.9.4

172.16.9.7

Finding VLSM Overlaps (
cont
)

Subnet

Subnet
Number

Broadcast
Address

First

172.16.0.0

172.16.1.255

Second

172.16.2.0

172.16.3.255

Third

172.16.4.0

172.16.5.255

Fourth

172.16.6.0

172.16.7.255

Fifth

172.16.8.0

172.16.9.255

Three subnets overlap

Subnet

Subnet
Number

Broadcast
Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.6.0

172.16.6.255

R1
-
R2
Serial

172.16.9.0

172.16.9.3

R1
-
R3
Serial

172.16.9.4

172.16.9.7

Finding VLSM Overlaps (
cont
)

Subnet

Subnet
Number

Broadcast
Address

First

172.16.0.0

172.16.1.255

Second

172.16.2.0

172.16.3.255

Third

172.16.4.0

172.16.5.255

Fourth

172.16.6.0

172.16.7.255

Fifth

172.16.8.0

172.16.9.255

We rule these out and choose the lowest value

(In this example)

Subnet

Subnet
Number

Broadcast
Address

R1

LAN

172.16.2.0

172.16.3.255

R2 LAN

172.16.4.0

172.16.5.255

R3 LAN

172.16.6.0

172.16.6.255

R1
-
R2
Serial

172.16.9.0

172.16.9.3

R1
-
R3
Serial

172.16.9.4

172.16.9.7

Designing a VLSM
subnetting

plan

1.
Determine minimum size mask needed
for each subnet

2.
Assign the largest subnet IDs

3.
Determine the next available address

4.
Assign the next largest subnet IDs

5.
Repeat steps 3 and 4 until all subnets
have an assigned ID

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
4 Requirements that Feed into a VLSM Design

Determine minimum size mask
needed for each subnet

For this example we need three masks

1.
/18: 14 host bits, 214


2 = 16,382
hosts/subnet

2.
/24: 8 host bits, 28


2 = 254
hosts/subnet

3.
/30: 2 host bits, 22


2 = 2 hosts/subnet

Assign the largest subnet
IDs
and Determine the next
available address


We need two /18 subnets so we assign
them to the two VLANs


We have two /18 subnets left over, starting
at 172.16.128.0

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
6 Four /18 Subnets Listed, with Two Allocated for Use

Assign the
next largest
subnet
IDs and Determine the next
available address


We need three /24 subnets so we assign
them to the three LANs attached to the
routers


We have 61 /24 subnets left over, starting
at 172.16.131.0

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
7 Subdividing 172.16.128.0/18 into 64 Subnets Using /24 Mask

Assign the
next largest
subnet
IDs and Determine the next
available address


We need three /30 subnets so we assign
them to the three WAN connections
between the routers


Since we have covered all the subnets we
are left with addresses from 172.16.131.12
to 172.16.255.255

From
CCNA ICND2 640
-
816 Official Cert Guide, Third Edition

by Wendell Odom, CCIE No. 1624

(ISBN: 1587204355) Copyright© 2012 Pearson Education, Inc.

Figure 5
-
8 The Three Allocated /24 Subnets and the Next Subnet to Divide Further