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Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  

Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…

VLSMs, Summarization, and Troubleshooting TCP/IP

VLSMs, Summarization, and Troubleshooting TCP/IP


Network Fundamentals
Image result for VLSMs, Summarization, and Troubleshooting TCP/IP
Apply troubleshooting methodologies to resolve problems

Perform fault isolation and document

Resolve or escalate

Verify and monitor resolution

Configure, verify, and troubleshoot IPv4 addressing and subnetting

Now that IP addressing and subnetting have been thoroughly covered in the last two chapters, you’re fully prepared and ready to learn all about variable length subnet masks (VLSMs). I’ll also show you how to design and implement a network using VLSM in this chapter. After ensuring you’ve mastered VLSM design and implementation, I’ll demonstrate how to summarize classful boundaries.
We’ll wrap up the chapter by going over IP address troubleshooting, focusing on the steps Cisco recommends to follow when troubleshooting an IP network.
So get psyched because this chapter will give you powerful tools to hone your knowledge of IP addressing and networking and seriously refine the important skills you’ve gained so far. So stay with me—I guarantee that your hard work will pay off! Ready? Let’s go 

Variable Length Subnet Masks (VLSMs)

Teaching you a simple way to create many networks from a large single network using subnet masks of different lengths in various kinds of network designs is what my primary focus will be in this chapter. Doing this is called VLSM networking, and it brings up another important subject I mentioned in Chapter 4, “Easy Sub-netting,” classful and classless networking.
Older routing protocols like Routing Information Protocol version 1 (RIPv1) do not have a field for subnet information, so the subnet information gets dropped. This means that if a router running RIP has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is called classful routing, and RIP is considered a classful routing protocol. Well cover RIP and the difference between classful and classless networks later on in Chapter 9, IP Routing,” but for now, just remember that if you try to mix and match subnet mask lengths in a network that’s running an old routing protocol, such as RIP, it just won’t work!
However, classless routing protocols do support the advertisement of subnet information, which means you can use VLSM with routing protocols such as RIPv2, Enhanced Interior Gateway Protocol (EIGRP), and Open Shortest Path First (OSPF). The benefit of this type of network is that it saves a bunch of IP address space.
As the name suggests, VLSMs can use subnet masks with different lengths for different router interfaces. Check out Figure 5.1 to see an example of why classful network designs are inefficient.

Typical classful network
you can see that there are two routers, each with two LANs and connected together with a WAN serial link. In a typical classful network design that’s running RIP, you could subnet a network like this: = Network (/28) = Mask
Our subnets would be—you know this part, right? 0, 16, 32, 48, 64, 80, etc., which allows us to assign 16 subnets to our inter-network. But how many hosts would be available on each network? Well, as you know by now, each subnet provides only 14 hosts, so each LAN has only 14 valid hosts available (don’t forget that the router interface needs an address too and is included in the amount of needed valid hosts). This means that one LAN doesn’t even have enough addresses needed for all the hosts, and this network as it is shown would not work as addressed in the figure!
Since the point-to-point WAN link also has 14 valid hosts, it would bgreat to be able to nick a few valid hosts from that WAN link to give to our LANs!
All hosts and router interfaces have the same subnet mask—again, known as classful routing—and if we want this network to be efficient, we would definitely need to add different masks to each router interface.
But that’s not our only problem—the link between the two routers will never use more than two valid hosts! This wastes valuable IP address space, and it’s the big reason you need to learn about VLSM network design.
VLSM Design
Let’s take Figure 5.1 and use a classless design instead, which will become the new network shown in Figure 5.2. In the previous example, we wasted address space—one LAN didn’t have enough addresses because every router interface and host used the same subnet mask. Not so good. A better solution would be to provide for only the needed number of hosts on each router interface, and were going to use VLSMs to achieve that goal.

 Classless network design
Now remember that we can use different size masks on each router interface. If we use a /30 on our WAN links and a /27, /28, and /29 on our LANs, well get 2 hosts per WAN interface and 30, 14, and 6 hosts per LAN interface—nice (remember to count your router interface as a host)! This makes a huge difference—not only can we get just the right amount of hosts on each LAN, we still have room to add more WANs and LANs using this same network!

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 5.1 shows you the block sizes used when creating VLSMs with Class C networks. For example, if you need 25 hosts, then youll need a block size of 32. If you need 11 hosts, youll use a block size of 16. Need 40 hosts?
Then youll need a block of 64. You cannot just make up block sizesthey’ve got to be the block sizes shown in Table 5.1. So memorize the block sizes in this table—it’s easy. They’re the same numbers we used with subnetting!
Table 5.1 Block sizes

Block Size

The next step is to create a VLSM table. Figure 5.3 shows you the table used in creating a VLSM network. The reason we use this table is so we don’t accidentally overlap networks.
Youll find the sheet shown in Figure 5.3 very valuable because it lists every block size you can use for a network address. Notice that the block sizes start at 4 and advance all the way up to a block size of 128. If you have two networks with block sizes of 128, you can have only 2 networks. With a block size of 64, you can have only 4, and so on, all the way to 64 networks using a block size of 4. Of course, this is assuming youre using the ip subnet-zero command in your network design.
So now all you need to do is fill in the chart in the lower-left corner, then add the subnets to the worksheet and you’re good to go!
Based on what youve learned so far about block sizes and the VLSM table, let’s create a VLSM network using a Class C network address for the network in Figure 5.4, then fill out the VLSM table
we have four WAN links and four LANs connected  together, so we need to create a VLSM network that will 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 

FIGURE 5.3 The VLSM table

VLSM network example 1

 VLSM table example 1
There are two important things to note here. The first is that we still have plenty of room for growth with this VLSM network design. The second point is that we could never achieve this goal with one subnet mask using classful routing.
Let’s do another one. shows a network with 11 networks, two block sizes of 64, one of 32, five of 16, and three of 4.

FIGURE 5.6 VLSM network example 2
First, create your VLSM table and use your block size chart to fill in the table with the subnets you need. Figure 5.7 shows a possible solution.
Notice that I filled in this entire chart and only have room for one more block size of 4. You can only gain that amount of address space savings with a VLSM network!
Keep in mind that it doesn’t matter where you start your block sizes as long as you always begin counting from zero. For example, if you had a block size of 16, you must start at 0 and incrementally progress from there—0, 16, 32, 48, and so on. You can’t start with a block size of 16 or some value like 40, and you can’t progress using anything but increments of 16.
Here’s another example. If you had block sizes of 32, start at zero like this: 0, 32, 64, 96, etc. Again, you don’t get to start wherever you want; you must always start counting from zero. In the example in Figure 5.7, I started at 64 and 128, with my two block sizes of 64. I didn’t have much choice because my options are 0, 64, 128, and 192. However, I added the block size of 32, 16, 8, and 4 elsewhere, but they were always in the correct increments required of the specific block size. Remember that if you always start with the largest blocks first, then make your way to the smaller blocks sizes, you will automatically fall on an increment boundary. It also guarantees that you are using your address space in the most effective way.
Okayyou have three locations you need to address, and the IP network you have received is to use as the addressing for the entire network. Youll use ip subnet-zero and RIPv2 as the routing protocol because RIPv2 supports VLSM networks but RIPv1 does not. Figure 5.8 shows the network diagram and the IP address of the Router A S0/0 interface.
VLSM table example 2

 VLSM design example 1
From the list of IP addresses on the right of the figure, which IP address do you think will be placed in each router’s Fast Ethernet 0/0 interface and serial 0/0 of Router B?
To answer this, look for clues in Figure 5.8. The first is that interface S0/0 on Router A has IP address assigned, which makes for an easy answer because A /30 is, which gives you a block size of 4. Your subnets are 0, 4, 8, etc. Since the known host has an IP address of 2, the only other valid host in the zero subnet is 1, so the third answer down is the right one for the S0/0 interface of Router B.
The next clues are the listed number of hosts for each of the LANs. Router A needs 7 hosts—a block size of 16 (/28). Router B needs 90 hosts
a block size of 128 (/25). And Router C needs 23 hosts—a block size of 32 (/27).
Figure 5.9 illustrates this solution.

FIGURE 5.9 Solution to VLSM design example 1
This is actually pretty simple because once you’ve figured out the block size needed for each LAN, all you need to get to the right solution is to identify proper clues and, of course, know your block sizes well!
One last example of VLSM design before we move on to summarization. Figure 5.10 shows three routers, all running RIPv2. Which Class C addressing scheme would you use to maintain the needs of this network while saving as much address space as possible?

FIGURE 5.10 VLSM design example 2
This is actually a pretty clean network design that’s just waiting for you to fill out the chart. There are block sizes of 64, 32, and 16 and two block sizes of 4. Coming up with the right solution should be a slam dunk! Take a look at my answer in Figure 5.11.

FIGURE 5.11 Solution to VLSM design example 2
My solution began at subnet 0, and I used the block size of 64. Clearly, I didn’t have to go with a block size of 64 because I could’ve chosen a block size of 4 instead. But I didn’t because I usually like to start with the largest block size and move to the smallest. With that done, I added the block sizes of 32 and 16 as well as the two block sizes of 4. This solution is optimal because it still leaves lots of room to add subnets to this network!
their existing network. There are no restrictions to prevent you from starting over with a completely new IP address scheme. Should you use a VLSM classless network or opt for a classful network?

Let’s say you happen to have plenty of address space because youre using the Class A private network address, so you really can’t imagine that you’d ever run out of IP addresses. So why would you want to bother with the VLSM design process in this environment?
Good question! Here’s your answer…
By creating contiguous blocks of addresses to specific areas of your network, you can then easily summarize the network and keep route updates with a routing protocol to a minimum. Why would anyone want to advertise hundreds of networks between buildings when you can just send one summary route between buildings and achieve the same result? This approach will optimize the network’s performance dramatically!
To make sure this is clear, let me take a second to explain summary routes. Summarization, also called super-netting, provides route updates in the most efficient way possible by advertising many routes in one advertisement instead of individually. This saves a ton of bandwidth and minimizes router processing. As always, you need to use blocks of addresses to configure your summary routes and watch your network’s performance hum along efficiently! And remember, block sizes are used in all sorts of networks anyway.
Still, it’s important to understand that summarization works only if you design your network properly. If you carelessly hand out IP subnets to any location on the network, you’ll quickly notice that you no longer have any summary boundaries. And you won’t get very far creating summary routes without those, so watch your step!

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 IP requires to parse the routing table when determining the best path to a remote network.

Figure 5.12 shows how a summary address would be used in an inter-network.

 Summary address used in an inter-network

Summarization is pretty straightforward because all you really need to have down is a solid understanding of the block sizes we’ve been using for 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, which will make it easy to find your answer: through network
Okay—so what’s the block size? Well, there are exactly 16 Class C networks, which fit neatly into a block size of 16.
Now that weve determined the block size, we just need to 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, we just need to figure out which mask will get us a block size of 16. If you came up with 240, you got it right! 240 would be placed in the third octet, which is exactly the octet where were summarizing, so the mask would be
Here’s another example:
Networks through
This isn’t as clean as the previous example because there are two possible answers. Here’s why: Since youre 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. Let’s explore your two options:
If you went with a block size of 16, then the network address would b172.16.32.0 with a mask of (240 provides a block of 16). The problem is that this only summarizes from 32 to 47, which means that networks 48 through 50 would be advertised as single networks. Even so, this could still be a good solution depending on your network design.

If you decided to go with a block size of 32 instead, 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 through 63 and we only have networks 32 to 50. No worries if youre planning on adding networks 51 to 63 later into the same network, but you could have serious problems in your inter network if somehow networks 51 to 63 were to show up and be advertised from somewhere else in your network! So even though this option does allow for growth, it’s a lot safer to go with option #1.
Let’s take a look at another example: Your summary address is, so 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 your network summary range is 144 to 159 in the third octet. This is why it comes in handy to be able to count in 16s!
A router with this summary address in the routing table will forward any packet having destination IP addresses of through
Only two more summarization examples, then we’ll move on to troubleshooting.
In summarization example 4, Figure 5.13, 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?

 Summarization example 4

No worries—solving this is easier than it looks initially. The question actually has the summary address listed in it: You already know that /20 is, which means youve got a block size of 16 in the third octet. Starting at 144, which is also right there in the question, makes the next block size of 16 equal 160. You can’t go above 159 in the third octet, so the IP addresses that will be forwarded are through
Okay, last one. In Figure 5.14, there are five networks connected to router R1. What’s the best summary address to R2?

 Summarization example 5
Ill be honest with you—this is a much harder question than the one in Figure 5.13, so youre going to have to look carefully to see the answer. A good approach here would be to write down all the networks and see if you can find anything in common with all of them:

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, meaning you would use a block size of 4 in the third octet. The IP addresses forwarded with this summary would be through
To summarize the summarization section, if you’ve nailed down your block sizes, then finding and applying summary addresses and masks is a relatively straightforward task. But youre going to get bogged down pretty quickly if you don’t know what a /20 is or if you can’t count by 16s!

Troubleshooting IP Addressing

Because running into trouble now and then in networking is a given, being able to troubleshoot IP addressing is clearly a vital skill. I’m not being negative here—just realistic. The positive side to this is that if you’re the one equipped with the tools to diagnose and clear up the inevitable trouble, you get to be the hero when you save the day! Even better? You can usually fix an IP network regardless of whether you’re on site or at home!
So this is where I’m going to show you the Cisco way” of troubleshooting IP addressing. Let’s use Figure 5.15 as an example of your basic IP trouble
—poor Sally can’t log in to the Windows server. Do you deal with this by calling the Microsoft team to tell them their server is a pile of junk and causing all your problems? Though tempting, a better approach is to first double-check and verify your network instead.
 Basic IP troubleshooting
Okay, let’s get started by going through the troubleshooting steps that Cisco recommends. They’re pretty simple, but important nonetheless. Pretend you’re at a customer host and they’re complaining that they can’t communicate to a server that just happens to be on a remote network.
Here are the four troubleshooting steps Cisco recommends:
Open a Command window and ping This is the diagnostic, or loopback, address, and if you get a successful ping, your IP stack is considered initialized. If it fails, then you have an IP stack failure and need to reinstall TCP/IP on the host.
Pinging with 32 bytes of data:
Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Ping statistics for
Packets: Sent &#x0003D; 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
From the Command window, ping the IP address of the local host (we’ll assume correct configuration here, but always check the IP configuration too!). 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 just 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.
Pinging with 32 bytes of data:
Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Ping statistics for
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
From the Command 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.

Pinging with 32 bytes of data:
Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Ping statistics for
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

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. You also know that the remote physical network is working.

Pinging with 32 bytes of data:
Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Reply from bytes=32 time<1ms TTL=128 Ping statistics for
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

If the user still can’t communicate with the server after steps 1 through 4 have been completed successfully, 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 commands that you can use to help troubleshoot your network from both a PC and a Cisco router. Keep in mind that though these commands may do the same thing, they’re implemented differently.
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 command prompt.
tracert Same function 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 function as arp -a, but displays the ARP table on a Cisco router. Like the commands traceroute and tracert, arp -a and show ip arp are not interchangeable through DOS and Cisco.
ipconfig /all Used only from a Windows command prompt; shows you the PC network configuration.
Once you’ve gone through all these steps and, if necessary, used the appropriate commands, what do you do when you find a problem? How do you go about fixing an IP address configuration error? Time to cover the next step—determining and fixing the issue at hand!
Determining IP Address Problems
It’s common for a host, router, or other network device to be configured with the wrong IP address, subnet mask, or default gateway. Because this happens way too often, you must know how to find and fix IP address configuration errors.
A good way to start is to draw out the network and IP addressing scheme. If that’s already been done, consider yourself lucky because though sensible, it’s rarely done. Even if it is, it’s usually outdated or inaccurate anyway. So either way, it’s a good idea to bite the bullet and start from scratch.
Once you have your network accurately drawn out, including the IP addressing scheme, you need to verify each host’s IP address, mask, and default gateway address to establish the problem. Of course, this is assuming that you don’t have a physical layer problem, or if you did, that you’ve already fixed it.

 IP address problem 1
A user in the sales department calls and tells you that she can’t get to ServerA in the marketing department. You ask her if she can get to ServerB in the marketing department, but she doesn’t know because she doesn’t have rights to log on to that server. What do you do?
First, guide your user through the four troubleshooting steps you learned in the preceding section. Okay—let’s say steps 1 through 3 work but step 4 fails. By looking at the figure, can you determine the problem? Look for clues in the network drawing. First, the WAN link between the Lab A router and the Lab B router shows the mask as a /27. You should already know that this mask is and determine that all networks are using this mask. The network address is What are our valid subnets and hosts? 256 224 = 32, so this makes our subnets 0, 32, 64, 96, 128, etc. So, by looking at the figure, you can see that subnet 32 is being used by the sales department. The WAN link is using subnet 96, and the marketing department is using subnet 64.

Now youve got to establish what the valid host ranges are for each subnet. From what you learned at the beginning of this chapter, you should now be able to easily determine the subnet address, broadcast addresses, and valid host ranges. The valid hosts for the Sales LAN are 33 through 62, and the broadcast address is 63 because the next subnet is 64, right? For the Marketing LAN, the valid hosts are 65 through 94 (broadcast 95), and for the WAN link, 97 through 126 (broadcast 127). By closely examining the figure, you can determine that the default gateway on the Lab B router is incorrect. That address is the broadcast address for subnet 64, so there’s no way it could be a valid host
Did you get all that? Let’s try another one to make sure. Figure 5.17 shows a network problem.

 IP address problem 2
A user in the Sales LAN can’t get to ServerB. You have the user run through the four basic troubleshooting steps and find that the host can communicate to the local network but not to the remote network. Find and define the IP addressing problem.
If you went through the same steps used to solve the last problem, you can see that first, the WAN link again provides the subnet mask to use—
/29, or Assuming classful addressing, you need to determine what the valid subnets, broadcast addresses, and valid host ranges are to solve this problem.
The 248 mask is a block size of 8 (256 248 = 8, as discussed in Chapter 4), so the subnets both start and increment in multiples of 8. By looking at the figure, you see that the Sales LAN is in the 24 subnet, the WAN is in the 40 subnet, and the Marketing LAN is in the 80 subnet. Can you see the problem yet? The valid host range for the Sales LAN is 25–30, and the configuration appears correct. The valid host range for the WAN link is 41–46, and this also appears correct. The valid host range for the 80 subnet is 81–86, with a broadcast address of 87 because the next subnet  is 88. Server B has been configured with the broadcast address of the subnet.

Okay, now that you can figure out misconfigured IP addresses on hosts, what do you do if a host doesn’t have an IP address and you need to assign one? What you need to do is scrutinize the other hosts on the LAN and figure out the network, mask, and default gateway. Let’s take a look at a couple of examples of how to find and apply valid IP addresses to hosts.
You need to assign a server and router IP addresses on a LAN. The subnet assigned on that segment is The router needs to be assigned the first usable address and the server needs the last valid host ID. What is the IP address, mask, and default gateway assigned to the server?
To answer this, you must know that a /29 is a mask, which provides a block size of 8. The subnet is known as 24, the next subnet in a block of 8 is 32, so the broadcast address of the 24 subnet is 31 and the valid host range is 25–30.
Server IP address: Server mask:
Default gateway: (router’s IP address) Take a look at Figure 5.18 and solve this problem.

 Find the valid host #1
Look at the router’s IP address on Ethernet 0. What IP address, subnet mask, and valid host range could be assigned to the host?
The IP address of the router’s Ethernet 0 is As you already know, a /27 is a 224 mask with a block size of 32. The router’s interface is in the 32 subnet. The next subnet is 64, so that makes the broadcast address of the 32 subnet 63 and the valid host range 33–62.
Host IP address:–62 (any address in the range except for 33, which is assigned to the router)
Default gateway:
Figure 5.19 shows two routers with Ethernet configurations already assigned. What are the host addresses and subnet masks of HostA and HostB?

 Find the valid host #2
Router A has an IP address of and Router B has an IP address of What are the host configurations? Router A Ethernet 0 is in the subnet and Router B Ethernet 0 is in the network.
Host A IP address:–126 Host A mask:
Host A default gateway: Host B IP address:–46 Host B mask:
Host B default gateway:
Just a couple more examples before you can put this chapter behind you
—hang in there!
Figure 5.20 shows two routers. You need to configure the S0/0 interface on RouterA. The IP address assigned to the serial link is What IP address can be assigned?

FIGURE 5.20 Find the valid host address #3
First, know that a /22 CIDR is, which makes a block size of 4 in the third octet. Since 17 is listed, the available range is 16.1 through 19.254, so in this example, the IP address S0/0 could be since that’s within the range.
Okay, last one! You need to find a classful network address that has one Class C network ID and you need to provide one usable subnet per city while allowing enough usable host addresses for each city specified in Figure 5.21. What is your mask?
FIGURE 5.21 Find the valid subnet mask
Actually, this is probably the easiest thing youve done all day! I count 5 subnets needed, and the Wyoming office needs 16 users—always look for the network that needs the most hosts! What block size is needed for the Wyoming office? Your answer is 32. You can’t use a block size of 16 because you always have to subtract 2. What mask provides you with a block size of 32? 224 is your answer because this provides 8 subnets, each with 30 hosts.
You’re done—the diva has sung and the chicken has safely crossed the road…whew! Time to take a break, but skip the shot and the beer if that’s what you had in mind because you need to have your head straight to go through the written lab and review questions next!


Again, if you got to this point without getting lost along the way a few times, you’re awesome, but if you did get lost, don’t stress because most people do! Just be patient with yourself and go back over the material that tripped you up until it’s all crystal clear. You’ll get there! This chapter provided you with keys to understanding the oh-so-very- important topic of variable length subnet masks. You should also know how to design and implement simple VLSM networks and be clear on summarization as well.
And make sure you understand and memorize Cisco’s troubleshooting methods. You must remember the four steps that Cisco recommends to take when trying to narrow down exactly where a network and/or IP addressing problem is and then know how to proceed systematically to fix it. In addition, you should be able to find valid IP addresses and subnet masks by looking at a network diagram.

Exam Essentials
Describe the benefits of variable length subnet masks (VLSMs). VLSMs enable the creation of subnets of specific sizes and allow the division of a classless network into smaller networks that do not need to be equal in size. This makes use of the address space more efficient because many times IP addresses are wasted with classful subnetting.
Understand the relationship between the subnet mask value and the resulting block size and the allowable IP addresses in each resulting subnet. The relationship between the classful network being subdivided and the subnet mask used determines the number of possible hosts or the block size. It also determines where each subnet begins and ends and which IP addresses cannot be assigned to a host within each subnet.

Describe the process of summarization or route aggregation and its relationship to subnetting. Summarization is the combining of subnets derived from a classful network for the purpose of advertising a single route to neighboring routers instead of multiple routes, reducing the size of routing tables and speeding the route process.
Calculate the summary mask that will advertise a single network representing all subnets. The network address used to advertise the summary address is always the first network address in the block of subnets. The mask is the subnet mask value that yields the same block size.
Remember the four diagnostic steps. The four simple steps that Cisco recommends for troubleshooting are ping the loop-back address, ping the NIC, ping the default gateway, and ping the remote device.
Identify and mitigate an IP addressing problem. Once you go through the four troubleshooting steps that Cisco recommends, you must be able to determine the IP addressing problem by drawing out the network and finding the valid and invalid hosts addressed in your network.
Understand the troubleshooting tools that you can use from your host and a Cisco router. The ping command tests your local IP stack, and tracert is a Windows command to track the path a packet takes through an inter-network to a destination. Cisco routers use the command traceroute, or just trace for short. Don’t confuse the Windows and Cisco commands. Although they produce the same output, they don’t work from the same prompts. The command ipconfig /all will display your PC network configuration from a DOS prompt, and arp
-a (again from a DOS prompt) will display IP-to-MAC-address mapping on a Windows PC.

Written Lab 5

In this section, you’ll complete the following lab to make sure you’ve got the information and concepts contained within them fully dialed in: Lab 5.1: Summarization Practice
You can find the answers to this lab in Appendix A, 
“Answers to Written Labs.”

Lab 5.1: Summarization Practice
For each of the following sets of networks, determine the summary address and the mask to be used that will summarize the subnets.
1. through
2. through
3. through
4. through
5. through
6. through
7. through
8. through
9. through
0. through


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