Transcript CCNA 3 Module 2 Introduction to Classless Routing

What Is VLSM and Why Is It Used?
1
Classful and Classless Routing
Classful (basic subnetting)
• Classful routing protocols require that a single network use
the same subnet mask
• RIP v1 required that all connected networks have the same
subnet mask
• So couldn’t subnet a subnet.
Classless (uses VLSM)
• VLSM allows a single autonomous system to have networks
with different subnet masks.
• Often referred to as "subnetting a subnet"
2
Support for Classless Routing
Open Shortest Path First (OSPF)
• Integrated Intermediate System to Intermediate
System (Integrated IS-IS)
• Enhanced Interior Gateway Routing Protocol
(EIGRP)
• RIP v2
• static routing
•
(but not RIP v1, IGRP, EGP)
3
A Waste of Space
4
Subnet Zero
• Not recommended by CISCO with normal subnetting
Due confusion when a network and a subnet have the same
addresses
• OK to use subnet zero with VLSM
Cisco IOS v12.0 routers use subnet zero by default
Or use the “no ip subnet-zero” command
• Not recommended by CISCO to use last subnet
Due confusion b/w a network broadcast and a subnet
directed broadcast
5
When to Use VLSM?
6
Wasting Host Addresses on WAN Connections
• Consider class C address 192.168.187.0/27
network
3 bits for subnet = 8 – 1
= 7 useable subnets
5 bits for hosts
= 30 hosts per subnet
• But we don’t want to waste 3 x 30 host subnets
on the WAN connections so we subnet one of the
subnets
• Must therefore have VLSM enabled routers
7
Using VLSM for WAN Connections
8
Subnetting Details
Subnet 6 (/27) further subnetted with mask of /30
to provide 8 subnets in total with 4 hosts per subnet (2 bits).
9
Further Example
• Could use 255.255.255.0 subnet mask - 172.16.0.0/24
255 networks with 255 hosts per network
But we would waste 253 hosts for WAN
• Similar problem to version 2 TCS using RIP V1
• So use /24 subnet mask and further subnet one subnet
with a /30 mask for the WAN
10
Further Example
• For example, consider the subnet address
172.16.32.0/20 and a network needing ten
host addresses.
• With this subnet address, there are over
4000 (212 – 2 = 4094) host addresses,
most of which will be wasted.
11
VLSM Solution
• Given 172.16.32.0/20
• Gives 16 subnets with 4096 hosts per subnet
Wasteful
• Further subnet one of the sixteen subnets into a
/26
• giving 64 subnets with 64 hosts
12
Subnet Details
13
A Working VLSM Example
14
Classless InterDomain Routing (CIDR) and VLSM
• Can only subnet a subnet that is empty
• Classless InterDomain Routing (CIDR) and VLSM
not only prevents address waste, but also
promotes route aggregation (OR summarization).
• Classless routing protocols carry a prefix that
consists of 32-bit IP address and bit mask in the
routing updates
15
Configuring VLSM
16
RIP History
• RIPv1 has the following limitations:
It does not send subnet mask information
in its updates.
It sends updates as broadcasts on
255.255.255.255.
It does not support authentication.
It is not able to support VLSM or classless
interdomain routing (CIDR).
17
RIPv2 Features
18
Comparing RIPv1 and RIPv2
19
Configuring RIPv2
20