Content Overview A network
administrator must anticipate and manage the physical growth of
a network, perhaps by buying or leasing another floor of the
building to house new networking equipment such as racks, patch
panels, switches, and routers. The network designer must choose
an addressing scheme that allows for growth. Variable-Length
Subnet Masking (VLSM) is a technique that allows for the
creation of efficient, scalable addressing schemes. With the
phenomenal growth of the Internet and TCP/IP, virtually every
enterprise must now implement an IP addressing scheme. Many
organizations select TCP/IP as the only routed protocol to run
on their network. Unfortunately, the architects of TCP/IP could
not have predicted that their protocol would eventually sustain
a global network of information, commerce, and entertainment.
Twenty years ago, IP version 4 (IPv4) offered an addressing
strategy that, although scalable for a time, resulted in an
inefficient allocation of addresses. IP version 6 (IPv6), with
virtually unlimited address space, is slowly being implemented
in select networks and may replace IPv4 as the dominant
protocol of the Internet. Over the past two decades, engineers
have successfully modified IPv4 so that it can survive the
exponential growth of the Internet. VLSM is one of the
modifications that has helped to bridge the gap between IPv4
and IPv6. Networks must be scalable in order to meet the
changing needs of users. When a network is scalable it is able
to grow in a logical, efficient, and cost-effective way. The
routing protocol used in a network does much to determine the
scalability of the network. Therefore, it is important that the
routing protocol be chosen wisely. Routing Information Protocol
(RIP) is still considered suitable for small networks, but is
not scalable to large networks because of inherent limitations.
To overcome these limitations yet maintain the simplicity of
RIP version 1 (RIP v1), RIP version 2 (RIP v2) was developed.
Students completing this module should be able to:
- Define VLSM and briefly describe the reasons for its
use
- Divide a major network into subnets of different
sizes using VLSM
- Define route aggregation and
summarization as they relate to VLSM
- Configure a
router using VLSM
- Identify the key features of RIP v1
and RIP v2
- Identify the important differences between
RIP v1 and RIP v2
- Configure RIP v2
- Verify and
troubleshoot RIP v2 operation
- Configure default routes
using the ip route and ip default-network
commands
Content 1.1 VLSM
1.1.1 What is VLSM and why is it used? As IP
subnets have grown, administrators have looked for ways to use
their address space more efficiently. One technique is called
Variable-Length Subnet Masks (VLSM). With VLSM, a network
administrator can use a long mask on networks with few hosts,
and a short mask on subnets with many hosts. In order to use
VLSM, a network administrator must use a routing protocol that
supports it. Cisco routers support VLSM with Open Shortest Path
First (OSPF), Integrated Intermediate System to Intermediate
System (Integrated IS-IS), Enhanced Interior Gateway Routing
Protocol (EIGRP), RIP v2, and static routing. VLSM allows an
organization to use more than one subnet mask within the same
network address space. Implementing VLSM is often referred to
as "subnetting a subnet", and can be used to maximize
addressing efficiency. Classful routing protocols require that
a single network use the same subnet mask. Therefore, network
192.168.187.0 must use just one subnet mask such as
255.255.255.0. VLSM is simply a feature that allows a single
autonomous system to have networks with different subnet masks.
If a routing protocol allows VLSM, use a 30-bit subnet mask on
network connections, 255.255.255.252, a 24-bit mask for user
networks, 255.255.255.0, or even a 22-bit mask, 255.255.252.0,
for networks with up to 1000 users.
Content 1.1
VLSM 1.1.2 A waste of space In the past,
it has been recommended that the first and last subnet not be
used. Use of the first subnet, known as subnet zero, for host
addressing was discouraged because of the confusion that can
occur when a network and a subnet have the same addresses. The
same was true with the use of the last subnet, known as the
all-ones subnet. It has always been true that these subnets
could be used. However, it was not a recommended practice. As
networking technologies have evolved, and IP address depletion
has become of real concern, it has become acceptable practice
to use the first and last subnets in a subnetted network in
conjunction with VLSM. In this network, the network management
team has decided to borrow three bits from the host portion of
the Class C address that has been selected for this addressing
scheme. If management decides to use subnet zero, it has eight
useable subnets. Each may support 30 hosts. If the management
decides to use the no ip subnet-zero command, it has
seven usable subnets with 30 hosts in each subnet. From Cisco
IOS version 12.0, remember that Cisco routers use subnet zero
by default. Therefore Sydney, Brisbane, Perth, and Melbourne
remote offices may each have 30 hosts. The team realizes that
it has to address the three point-to-point WAN links between
Sydney, Brisbane, Perth, and Melbourne. If the team uses the
three remaining subnets for the WAN links, it will have used
all of the available addresses and have no room for growth. The
team will also have wasted the 28 host addresses from each
subnet to simply address three point-to-point networks. Using
this addressing scheme one third of the potential address space
will have been wasted. Such an addressing scheme is fine for a
small LAN. However, this addressing scheme is extremely
wasteful if using point-to-point connections.
Content
1.1 VLSM 1.1.3 When to use
VLSM? It is important to design an addressing scheme that
allows for growth and does not involve wasting addresses. This
section examines how VLSM can be used to prevent waste of
addresses on point-to-point links.This time the networking team
decided to avoid their wasteful use of the /27 mask on the
point-to-point links. The team decided to apply VLSM to the
addressing problem. To apply VLSM to the addressing problem,
the team will break the Class C address into subnets of
variable sizes. Large subnets are created for addressing LANs.
Very small subnets are created for WAN links and other special
cases. A 30-bit mask is used to create subnets with only two
valid host addresses. In this case this is the best solution
for the point-to-point connections. The team will take one of
the three subnets they had previously decided to assign to the
WAN links, and subnet it again with a 30-bit mask. In the
example, the team has taken one of the last three subnets,
subnet 6, and subnetted it again. This time the team uses a
30-bit mask. Figures and illustrate that after using VLSM, the
team has eight ranges of addresses to be used for the
point-to-point links.
Content 1.1 VLSM
1.1.4 Calculating subnets with VLSM VLSM helps to
manage IP addresses. VLSM allows for the setting of a subnet
mask that suits the link or the segment requirements. A subnet
mask should satisfy the requirements of a LAN with one subnet
mask and the requirements of a point-to-point WAN with another.
Look at the example in Figure which illustrates how to
calculate subnets with VLSM. The example contains a Class B
address of 172.16.0.0 and two LANs that require at least 250
hosts each. If the routers are using a classful routing
protocol the WAN link would need to be a subnet of the same
Class B network, assuming that the administrator is not using
IP unnumbered. Classful routing protocols such as RIP v1, IGRP,
and EGP are not capable of supporting VLSM. Without VLSM, the
WAN link would have to have the same subnet mask as the LAN
segments. A 24-bit mask (255.255.255.0) would support 250
hosts. The WAN link only needs two addresses, one for each