VLSMs, Summarization, and Troubleshooting

(Chapter 5)

April 11, 2023

VLSM• Subnetting involves dividing a range of host

addresses into multiple blocks

IP Host Range (192.168.1.x)

.0 .1 .2 … … .253 .254 .255

VLSM• Variable length subnet masks (VLSMs) allow

for different sized subnets (i.e., different subnet masks) across a network

• Advantage: they use address space more efficiently

VLSM

VLSMNetwork: 192.168.1.0

58 Hosts 12 Hosts

9 Hosts 29 Hosts

2 Hosts

Network: 192.168.1.0

58 12

9 29

2

Network Hosts Block Subnet Mask

A

B

C

D

E

Block Size: 64

Block Size: 32

Block Size: 16

Block Size: 4

Summarization• Also called route aggregation or supernetting• Allows multiple networks to be described

using one address. Why?– Less data transferred between routers– Easier processing for each router

Summarization• Things to consider:

– Find the appropriate block size (i.e., subnet mask)– Don’t include addresses that aren’t there– Summary route is the network address

SummarizationExample #1

Summarize networks 172.16.32.0 through 172.16.50.0

Option A Option B

Summary Address 172.16.32.0 172.16.32.0

Summary Mask 255.255.240.0 255.255.224.0

Block Size 16 32

SummarizationExample #2

Summarize networks:172.1.4.128/25172.1.7.0/24172.1.6.0/24172.1.5.0/24172.1.4.0/25

Summary Address 172.1.4.0

Summary Mask 255.255.252.0

Block Size 4