Nov 14, 2011

Abstract of CCNA study guide-11 - VLSM

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.

Block Size

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 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, 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: through network
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, 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

Here’s another example: Networks through
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 with a mask of (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, but the mask would be (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—what’s the range of host addresses that would be forwarded according to this summary? The /20 provides a summary address of and mask of 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 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: You already know that /20 is, 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 through (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:
_  ,  ,  ,  ,
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 using a mask of,which means you will use a block size of 4 in the third octet. The IP addresses forwarded with this summary are through
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 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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