Information Technology Reference
In-Depth Information
Classless Routing Protocol
Classless routing protocols include
RIPv2
,
EIGRP
,
OSPF
,
IS
-
IS
, and
BGP
. These routing protocols
include the
subnet mask
with the network address in their routing updates.
VLSM
In a previous course, you learned how variable-length subnet masking (VLSM) allows the use of dif-
ferent masks for each subnet. After a network address is subnetted, those subnets can be further sub-
netted. As you most likely recall, VLSM is simply subnetting a subnet. VLSM can be thought of as
sub-subnetting.
In the following sections, you will work through four VLSM exercises and six VLSM scenarios.
VLSM Addressing Design Exercises
In the following VLSM addressing design exercises, you will apply your VLSM addressing skills to a
three-router topology. Each exercise is progressively more difficult than the previous one. There might
be more than one correct answer in some situations. However, you should always practice good
addressing design by assigning your subnets contiguously. This will allow the summary of a group of
subnets into one aggregate route, thus decreasing the size of routing tables.
Exercise 1
Assume that 4 bits were borrowed from the host portion of 192.168.1.0/24. You are
not
using VLSM.
Starting with subnet 0, label Figure 6-1 contiguously with subnets. Start with the LAN on RTA and
proceed clockwise.
Figure 6-1
Addressing Design Exercise 1 Topology: Subnets (Answer)
192.168.1.0/28
Address Space
192.168.1.0/24
RTA
192.168.1.80/28
192.168.1.16/28
192.168.1.64/28
192.168.1.32/28
RTC
RTB
192.168.1.48/28
How many TOTAL valid host addresses will be wasted on the WAN links?
We assigned three WAN subnets with 14 hosts each. Two hosts are used; therefore, 12 hosts x 3 WAN
subnets = 36 wasted host addresses.
Now come up with a better addressing scheme using VLSM. Start with the same 4 bits borrowed
from the host portion of 192.168.1.0/24. Label each of the LANs with a subnet. Then subnet the next
available subnet to provide WAN subnets without wasting any host addresses. Label Figure 6-2 with
the subnets.
