Continue the series of Abstract CCNA study guide book .
Variable Length Subnet Masks (VLSMs)
I’m going to show you a
simple way to take one network and create many networks using subnet masks of
different lengths on different types of network designs. This is called VLSM
networking, and it does bring up another subject I mentioned at the beginning
of this chapter: classful and classless networking.
All hosts and router
interfaces have the same subnet mask, this is called classful routing. And if
we want this network to be more efficient, we definitely need to add different
masks to each router interface and this is called classless routing.
we use what are referred
to as Variable Length Subnet Masks (VLSMs) to provide only the needed number of
hosts on each router interface
Implementing VLSM Networks
To create VLSMs quickly
and efficiently, you need to understand how block sizes and charts work
together to create the VLSM masks. Table 3.3 shows the block sizes used when creating
VLSMs with Class C networks.
Prefix
|
Mask
|
Hosts
|
Block Size
|
Subnets
|
/25
|
128
|
126
|
128
|
2
|
/26
|
192
|
62
|
64
|
4
|
/27
|
224
|
30
|
32
|
8
|
/28
|
240
|
14
|
16
|
16
|
/29
|
248
|
6
|
8
|
32
|
/30
|
252
|
2
|
4
|
64
|
For example, if you need
25 hosts, then you’ll need a block size of 32. If you need 11 hosts, you’ll use
a block size of 16. Need 40 hosts? Then you’ll need a block of 64.
The next step is to
create a VLSM table. Next Figure shows
you the table used in creating a VLSM network. The reason we use this table is
so we don’t accidentally overlap networks.
Notice that the block
sizes are listed starting from a block size of 4 all the way to a block size of
128. If you have two networks with block sizes of 128, you’ll quickly see that
you can have only two networks. With a block size of 64, you can have only four
networks, and so on.
Now, just fill in the
chart in the lower-left corner, and then add the subnets to the worksheet and
you’re good to go.
So let’s take what we’ve
learned so far about our block sizes and VLSM table and create a VLSM using a
Class C network address 192.168.10.0 for the network in Figure below. Then fill
out the VLSM table, as shown in Figure .
In Figure 3.6, we have
four WAN links and four LANs connected together. We need to create a VLSM
network that will allow us to save address space. Looks like we have two block
sizes of 32, a block size of 16, and a block size of 8, and our WANs each have
a block size of 4. Take a look and see how I filled out our VLSM chart in Figure
below .
Summarization
Summarization, also
called route aggregation, allows routing protocols to advertise many networks as
one address. The purpose of this is to reduce the size of routing tables on
routers to save memory, which also shortens the amount of time for IP to parse
the routing table and find the path to a remote network.
Next Figure shows how a summary address would be used in
an internetwork.
all you need to have down are the block sizes
that we just used in learning subnetting and VLSM design. For example, if you wanted
to summarize the following networks into one network advertisement, you just
have
to find the block size
first; then you can easily find your answer: 192.168.16.0 through network
192.168.31.0
What’s the block size?
There are exactly 16 Class C networks, so this neatly fits into a block size of
16.
Okay, now that you know
the block size, you can find the network address and mask used to summarize
these networks into one advertisement. The network address used to advertise the
summary address is always the first network address in the block—in this
example, 192.168.16.0. To figure out a summary mask, in this same example, what
mask is used to get a block size of 16? Yes, 240 is correct. This 240 would be
placed in the third octet—the octet where we are summarizing. So, the mask
would be 255.255.240.0.
Here’s another example: Networks
172.16.32.0 through 172.16.50.0
This is not as clean as
the previous example because there are two possible answers, and here’s why:
Since you’re starting at network 32, your options for block sizes are 4, 8, 16,
32,64, etc., and block sizes of 16 and 32 could work as this summary address.
_ Answer #1: If
you used a block size of 16, then the network address is 172.16.32.0 with a
mask of 255.255.240.0 (240 provides a block of 16). However, this only
summarizes from 32 to 47, which means that networks 48 through 50 would be
advertised as single networks. This is probably the best answer, but that
depends on your network design.
Let’s look at the next
answer.
_ Answer #2: If
you used a block size of 32, then your summary address would still be 172.16.32.0,
but the mask would be 255.255.224.0 (224 provides a block of 32). The possible
problem with this answer is that it will summarize networks 32 to 63 and we
only have networks 32 to 50. No worries if you’re planning on adding networks
51 to 63 later into the same network, but you could have serious problems in
your internetwork if somehow networks 51 to 63 were to show up and be
advertised from somewhere else in your
network! This is the
reason why answer number one is the safest answer.
Let’s take a look at
another example, but let’s look at it from a host’s perspective.
Your summary address is
192.168.144.0/20—what’s the range of host addresses that would be forwarded
according to this summary? The /20 provides a summary address of 192.168.144.0
and mask of 255.255.240.0. The third octet has a block size of 16, and starting
at summary address 144, the next block
of 16 is 160, so our
network summary range is 144 to 159 in the third octet (again, you must
be able to count in
16s!).
Figure below , the
Ethernet networks connected to router R1 are being summarized to R2 as 192.168.144.0/20. Which range of IP addresses
will R2 forward to R1 according to this summary?
No worries—this is really
an easier question than it looks. The question actually has the summary address
listed: 192.168.144.0/20. You already know that /20 is 255.255.240.0, which
means you’ve got a block size of 16 in the third octet. Starting at 144 (this
is also right there in the question), the next block size of 16 is 160, so you
can’t go above 159 in the third octet. The IP addresses that will be forwarded
are 192.168.144.1 through 192.168.159.255. (Yes, the broadcast address is
forwarded.)
Okay, last one. In next Figure,
there are five networks connected to router R1. What’s the best summary address
to R2?
I’m going to be
honest—this is a much harder question than the one in Figure 3.15. You’re going
to have to look pretty hard to see the answer. The first thing to do with this
is to write down all the networks and see if you can find anything in common
with all six:
_ 172.1.4.128/25 , 172.1.7.0/24
, 172.1.6.0/24
, 172.1.5.0/24 , 172.1.4.0/24
Do you see an octet that
looks interesting to you? I do. It’s the third octet. 4, 5, 6, 7, and yes, it’s
a block size of 4. So you can summarize 172.1.4.0 using a mask of
255.255.252.0,which means you will use a block size of 4 in the third octet.
The IP addresses forwarded with this summary are 172.1.4.1 through 172.1.7.255.
Troubleshooting IP Addressing
Let’s use next Figure as an
example of your basic IP trouble—poor Sally can’t log in to the Windows server.
Here are the four troubleshooting steps Cisco
recommends:
1. Open a
DOS window and ping 127.0.0.1. This is the diagnostic, or loopback, address, and
if you get a successful ping, your IP stack is considered to be initialized. If
it fails, then you have an IP stack failure and need to reinstall TCP/IP on the
host.
2. From the
DOS window, ping the IP address of the local host. If that’s successful, your
network interface card (NIC) is functioning. If it fails, there is a problem
with the NIC. Success here doesn’t mean that a cable is plugged into the NIC,
only that the IP protocol stack on the host can communicate to the NIC (via the
LAN driver).
3. From the
DOS window, ping the default gateway (router). If the ping works, it means that
the NIC is plugged into the network and can communicate on the local network.
If it fails, you have a local physical network problem that could be anywhere
from the NIC to the router.
4. If steps
1 through 3 were successful, try to ping the remote server. If that works, then
you know that you have IP communication between the local host and the remote
server
If the user still can’t communicate with the server after steps 1
through 4 are successful, you probably have some type of name resolution
problem and need to check your Domain Name System (DNS) settings. But if the
ping to the remote server fails, then you know you have some type of remote
physical network problem and need to go to the server and work through steps 1 through
3 until you find the snag.
Before we move on to determining IP address problems and how to fix
them, I just want to mention some basic DOS commands that you can use to help
troubleshoot your network from both a PC and a Cisco router .
Packet InterNet Groper (ping) Uses ICMP echo request and replies to test if a node
IP stack is initialized and alive on the network.
traceroute Displays
the list of routers on a path to a network destination by using TTL time-outs
and ICMP error messages. This command will not work from a DOS prompt.
tracert Same
as traceroute, but it’s a Microsoft Windows command and will not work on a
Cisco router.
arp -a Displays
IP-to-MAC-address mappings on a Windows PC.
show ip arp Same
command as arp -a, but displays the ARP table on a Cisco router.
ipconfig /all Used
only from a DOS prompt, shows you the PC network configuration.
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