router. Therefore there would be 252 addresses
wasted. If VLSM were used in this example, a 24-bit mask would
still work on the LAN segments for the 250 hosts. A 30-bit mask
could be used for the WAN link because only two host addresses
are needed. In Figure the subnet addresses used are those
generated from subdividing the 172.16.32.0/20 subnet into
multiple /26 subnets. The figure illustrates where the subnet
addresses can be applied, depending on the number of host
requirements. For example, the WAN links use subnet addresses
with a prefix of /30. This prefix allows for only two hosts,
just enough hosts for a point-to-point connection between a
pair of routers. To calculate the subnet addresses used on the
WAN links, further subnet one of the unused /26 subnets. In
this example, 172.16.33.0/26 is further subnetted with a prefix
of /30. This provides four more subnet bits and therefore 16
(24) subnets for the WANs. Figure illustrates how to work
through a VLSM masking system. VLSM allows the subnetting of an
already subnetted address. 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. With
VLSM it is possible to further subnet the address
172.16.32.0/20 to give more network addresses and fewer hosts
per network. For example, by subnetting 172.16.32.0/20 to
172.16.32.0/26, there is a gain of 64 (26) subnets, each of
which could support 62 (26 – 2) hosts. Use this procedure to
further subnet 172.16.32.0/20 to 172.16.32.0/26: Step 1
Write 172.16.32.0 in binary form. Step 2 Draw a vertical
line between the 20th and 21st bits, as shown in Figure . /20
was the original subnet boundary. Step 3 Draw a vertical
line between the 26th and 27th bits, as shown in Figure . The
original /20 subnet boundary is extended six bits to the right,
becoming /26. Step 4 Calculate the 64 subnet addresses
using the bits between the two vertical lines, from lowest to
highest in value. The figure shows the first five subnets
available. It is important to remember that only unused subnets
can be further subnetted. If any address from a subnet is used,
that subnet cannot be further subnetted. In the example, four
subnet numbers are used on the LANs. Another unused subnet,
172.16.33.0/26, is further subnetted for use on the WANs.
Lab Activity Lab Exercise: Calculating VLSM Subnets In this
lab, students will use variable-length subnet mask (VLSM) to
support more efficient use of the assigned IP addresses and to
reduce the amount of routing information at the top level.
Content 1.1 VLSM 1.1.5 Route
aggregation with VLSM When using VLSM, try to keep the
subnetwork numbers grouped together in the network to allow for
aggregation. This means keeping networks like 172.16.14.0 and
172.16.15.0 near one another so that the routers need only
carry a route for 172.16.14.0/23. The use of Classless
InterDomain Routing (CIDR) and VLSM not only prevents address
waste, but also promotes route aggregation, or summarization.
Without route summarization, Internet backbone routing would
likely have collapsed sometime before 1997. Figure illustrates
how route summarization reduces the burden on upstream routers.
This complex hierarchy of variable-sized networks and
subnetworks is summarized at various points, using a prefix
address, until the entire network is advertised as a single
aggregate route, 200.199.48.0/22. Route summarization, or
supernetting, is only possible if the routers of a network run
a classless routing protocol, such as OSPF or EIGRP. Classless
routing protocols carry a prefix that consists of 32-bit IP
address and bit mask in the routing updates. In Figure , the
summary route that eventually reaches the provider contains a
20-bit prefix common to all of the addresses in the
organization, 200.199.48.0/22 or 11001000.11000111.0011. For
summarization to work properly, carefully assign addresses in a
hierarchical fashion so that summarized addresses will share
the same high-order bits. Remember the following rules:
- A router must know in detail the subnet numbers attached to
it.
- A router does not need to tell other routers
about each individual subnet if the router can send one
aggregate route for a set of routers.
- A router using
aggregate routes would have fewer entries in its routing
table.
VLSM allows for the summarization of routes
and increases flexibly by basing the summarization entirely on
the higher-order bits shared on the left, even if the networks
are not contiguous. The graphic shows that the addresses, or
routes, share each bit up to and including the 20th bit. These
bits are colored red. The 21st bit is not the same for all the
routes. Therefore the prefix for the summary route will be 20
bits long. This is used to calculate the network number of the
summary route. Figure shows that the addresses, or routes,
share each bit up to and including the 21st bit. These bits are
colored red. The 22nd bit is not the same for all the routes.
Therefore the prefix for the summary route will be 21 bits
long. This is used to calculate the network number of the
summary route. Web Links Internetworking Design Basics
http://www.cisco.com/en/US/products/sw/
iosswrel/
ps1828/products_configuration_
guide_
chapter09186a00800ca569.html Configuring IP Routing
Protocol-Independent Features http://www.cisco.com/en/US/products/sw/
iosswrel/ ps1835/products_configuration_
guide_chapter09186a00800ca765.html
#xtocid2
Content
1.1 VLSM 1.1.6 Configuring VLSM
If VLSM is the scheme chosen, it must then be calculated and
configured correctly. In this example allow for the following:
Network address: 192.168.10.0 The Perth router has to support
60 hosts. In this case, a minimum of six bits are needed in the
host portion of the address. Six bits will yield 62 possible
host addresses, 26 = 64 – 2 = 62, so the division was
192.168.10.0/26. The Sydney and Singapore routers have to
support 12 hosts each. In these cases, a minimum of four bits
are needed in the host portion of the address. Four bits will
yield 14 possible host addresses, 24 = 16 – 2 = 14, so the
division is 192.168.10.96/28 for Sydney and 192.168.10.112/28
for Singapore. The Kuala Lumpur router requires 28 hosts. In
this case, a minimum of five bits are needed in the host
portion of the address. Five bits will yield 30 possible host
addresses, 25 = 32 – 2 = 30, so the division here is
192.168.10.64/27. The following are the point-to-point
connections: - Perth to Kuala Lumpur
192.168.10.128/30 – Since only two addresses are required,
a minimum of two bits are needed in the host portion of the
address. Two bits will yield two possible host addresses (22 =
4 – 2 = 2) so the division here is 192.168.10.128/30.
- Sydney to Kuala Lumpur 192.168.10.132/30 – Since
only two addresses are required, a minimum of two bits are
needed in the host portion of the address. Two bits will yield
two possible host addresses (22 = 4 – 2 = 2) so the division
here is 192.168.10.132/30.
- Singapore to Kuala
Lumpur 192.168.10.136/30 – Since only two addresses are
required, a minimum of two bits are needed in the host portion
of the address. Two bits will yield two possible host addresses
(22 = 4 – 2 = 2) so the division here is 192.168.10.136/30.
There is sufficient host address space for two host