Starting Packet Tracer (PT from now on), we are in Logical workspace that is visible in the up-left corner with the word Logical and the icon here on the left. There are two workspaces: Logical and Physical, with the Logical being the most frecuently used.

The logical workspace is the one we use to build topologies, we just need to select a group of elements from the panel at the bottom, the most to the left, which have routers, switches, hubs, wireless, connections, end devices, WAN emulation, Customized elements and Multiuser cloud. The name of the group of elements appear in the center of the panel. Next to it, depending on the group selected are placed the individual elements, we select anyone we need and place it on the logical workingspace. An average topology, like the one I’m going to build here, have routers, switches, end devices and links.

The other workspace is Physical. This one appears when we click the gray icon “behind” the icon of Logical workspace also in the upper-left corner. This space seem to be a growing part of PT, by now, the physical workspace only allow the visualization of a region, and after some clicks more, the rack and a physical disposition of the equipment placed in the logical workingspace. I think that this workingspace is not finished and should support to deploying the devices in different cities and buildings.

Operational modes

The starting point is also in real time mode, which means that all we do happens in the time it happens in real time as with real devices. The mode is indicated by the icon in the bottom-right corner. When we are on this mode, if we place a packet in the topology the only thing we see is a blinking green dot in the links involved in transmitting this packet, and that takes less than a second as it would be with real devices.

One simple test we could use is add a simple PDU, from the right side panel. This test is equivalent as sending a unique ping packet through the network which takes source addresses from the first device we click on when cursor is an envelope with a plus sign, the takes destination addresses is taken from the second selected device. After we do all this, a new line in the scenarios panel appear -the last panel in the bottom at the right-. The options in the scenarios panel are fire (again), last status, Source device, Destination device, type, Color, Time (sec), Periodic, Num, Edit, Delete. For now the only important options for us are fire, edit and delete, the first sends again the packet, edit allows change packet parameters and is a little advanced for us now and finally delete a particular packet when we double click in the delete option for a packet corresponding line. If someone ask for the delete and new buttons, they are to add and delete scenarios, which are groups of packets and is a topic for a future post.

The other operational mode is Simulation. The simulation mode allows for watching the way packets are transmitted from device to device and see how the packets are manipulated for each device in the path to the destnation. This mode is one of the most useful ones, but for now we are going to explore just real time mode.

Toolbar and special indicators

PT interface is very intuitive, the toolbar is the classical icons below the menu bar. The only two non common icons are Activity Wizard, Custom Devices y Palette but today I describe just Palette. This function allows us to draw shapes around devices, for example squares or circles around part of a topology, the drawings doesn’t have any relation with the way the network works and is specially useful when we write some text in a label (right side panel).

From the right side panel, only delete icon is important. When we click on it, if there are a selected element (usually it is) the PT ask for its deletion, so the recommended way of delete an element is when nothing in the topology is selected. When we click on delete icon, the cursor becomes an X with a hole and anything in the hole is deleted when we click on it, afterwards the cursor returns to select funtion.

GUI device configuration

PT allows for many configuration options, for example command line interface for any device and support almost all the commands to study CCNA, but starting we are going to explore the graphical user interface, in which we don’t need to know nothing about IOS or Cisco device configuration, just basic networking. The best way to learn is watching to do, so I’ll build a topology with a point to point network of two PCs connected by a crossover cable, below them I’ll build a complete topology with 2 PCs, 1 Switch, 1 Router and 1 Server. I’ll configure everything to work, that’s not important, look the way to configure devices with the GUI. Note also that when I keep the cursor above something a hint pops up with relevant minimal information.

Conclusions

PT opens a new world of easy networking that couldn’t be easier. We’ve touched just the surface, the most basic elements and even though we can build working topologies, we need to explore much more and that what we’re going to do in future posts. Be patient and keep visiting.

Note that there are some other indicators I didn’t write about, for example, when you navigate the physical workspace there are a NAVIGATE indicator in the bar below the tool bar, there are others we’ll explore in future posts.

Finally, we must agree that PT makes easy something usually very hard: learning networking. I hope you enjoy all this work and keep comming to read more.

[Original post on spanish]

What is Packet Tracer?

First I ‘ve got to say, PT stands for Packet Tracer. The PT is closely tied with the Cisco networking academies, it’s an application that allows to design network topologies with the same icons used in the official CCNA curriculum. Beyond being able to design the topologies, the PT allows you to configure almost all the technologies that are mentioned in the curricula and to observe how they work as if they were real equipment. Up to today, the latest version is 5.1, recently released. If we had to define PT in short it would be simulator of data networks.

The initial objective of PT is to be a didactic tool, but after version 5.0, the simulation capacity is such that can serve to preconfigure a real network or asses if some experimental implementation can be viable. Anyway it is necessary to remember that that is not the PT main objective and therefore you can’t take seriously a test for some real implementation, for such a purpose it’s better to design prototype topologies, to try with the real equipment in controlled topologies or to use emulators with the same amount of caution.

What can be done with PT?

PT allows, as I already said, to design topologies with the same icons used in the CCNA curriculum, which facilitates its comprehension. The equipment has real references and its interface is so realistic that if you need to change some in the physical configuration you need to shut the device off before the change. Other characteristics of realism of the PT the multiple ways to visualize the topology, among them, the physical view whose use shows a map of some city (could be San Francisco?) within it the office and within the office the wiring closet. If we go deep enough in the physical view to click the wiring closet it shows a rack with the equipment that we have in the topology as they would be seen actually… and we could even shut down any from there! (although we could only do that). Apparently the physical space is unfinished but it devise how thigs are going to be, additionally is possible to divide the physical space in different closets, cities or buildings, I can figure out that as a trend to future possibility to distribute the topology in geographically separated spaces like a real topology.

Being a little more pragmatic (not so didactic), the PT allows you to access each device of the topology and to configure it, be it by a very intuitive GUI -graphical user interface-  or by command line interface (CLI) as we would do with real equipment. The PT is flexible enough, to have desktop PCs with a real desktop with common applications used in every day the network: a browser and command  console, additionally, some other tools that we will use ordinarily: telnet console, terminal emulator (like hyperterminal or minicom) and configuration interfaces for dial-up access, wireless network and wired network. Is there also the option to add servers which can execute services like HTTP, DNS and TFTP that we could connect to the network to simulate transactions, let’s say, from the navigators of the client PCs or to save configurations from networking equipment.

There are an intensively used feature to visualize flow of packages called simulatin view. The idea is to see and control how packets are created and destroyed after some event is triggered, e.g. doing ping or trying to see a web page stored in one of the servers of the topology from the navigator on one of the PC.  Should be the case fail or success, we can see the  processes running on every packet in each of the devices it’s been through in the terms of OSI reference model. In the case of failure we can watch which was the last process applied to the packet and read the actual cause of the failure.

Advanced Features  in Packet Tracer

For me all described features are quite advanced, but for more sophistication, in the same CCNA Exploration curriculum I’ve seen laboratories with more than 20 devices: 10 routers, 10 switches and like 10 PCs including PCs with wireless cards in use, in other words, the power of PT simulation is sufficiently great for the CCNA scope, in fact, some people say that PT can be used in CCNP courses. A characteristic that goes of the hand with those big laboratories is the logical grouping of devices or clustering. By means of clustering several devices in a cloud, they can be treated as a single device (an icon).

One of the most used features in the curriculum is the activities, they consist of laboratories with embeded instructions which even keep a registry of the percentage of the tasks that it needs to be done for the activity to be completed, permanently showing the percentage of activity completion. Additionally to the instructions and the control of the completed activity percentage, the activities can restrict some regular options, for example, you can prevent the use of the graphical interface in a server or to restrict the configuration of a router to just the console, blocking the use of graphical interface to do the same, thus forcing a student to use CLI (Command Line Interface). The activities are not exclusive of the curriculum, the PT has wizards to create activities, allowing any person, hopefully instructors, to make their own activities.

Another powerful characteristic from PT 5.0 is the multiuser extension, that allows to develop laboratories from different computers, that is to say, an activity can be distributed in two or more PCs (each for a different student) connected by network to configure the topology designed for the activity. It even exists additional material available for the instructors in the Academy Connection that distributes a quite big laboratory in 6 different collaborative laboratories with supervision from the instructor. In a much more simple context, I have used it to task my students with the most complex laboratories in the curriculum and to urge them to develop it in a team using the multiuser extension, any other way would be very difficult to finish on time.

A characteristic new to PT 5,1, is the possibility of adding extensions of third parties called External Appplications, that is to say, PT5.1 publishes an API to interact with it, in such a way anyone wanting to develop an application that uses PT capabilities can do it and install it like an extension, giving new functionalities to the PT.

How to obtain the Packet Tracer?

As usual with Cisco, this is a delicate issue. Packet Tracer, as already told, is closely tied with cisco networking academies and that is its unique aim. It has a user license that restrict its usage to just academy related activities. The PT can be downloaded free of charge from Academy Connection itself, if you are instructor, student or alumni (graduated student who created an alumni profile) of a networking academy, after log-in, in the left and below all options there will be an icon to download the latest version. If there are two alternatives, one is tutorial included and the other without it. The tutorial is a litle big but very useful, it’s composed by small videos showing how to use each feature of the program. I’ve seen other alternatives for download but, not recommendable, one never knows what are we going to stumble into.

Conclusions

PT is a very powerful tool, didactic power (its primary objective) as much as technical utility and power. It gives us the possibility that each student learns at his/her own pace and to experiment live how the technologies work. To the instructors it gives thousands of ways to illustrate the concepts and to generate activities that are stimulating and illustrative for their students.

As usual, I am became to excited and I forgot I am not talking but writing, and people get tired to read so much. I only hope to rise enough enthusiasm with regard to the PT and that you’ll continue visiting this blog to follow the sequence. All the mentioned features and capabilities are going to be explored in detail and probably with some illustrative videos.

The task

Given the following network address 192.168.0.0/24, design an addressing scheme to tackle the next requirements using VLSM, that means, optimizing the addressing space.

1. A subnet for teachers which will have at least 20 hosts
2. A subnet for students which will have at least 80 hosts
3. A subnet for guests which will have at least 20 hosts
4. Three subnets of 2 hosts for the connection between routers

Solution

First order the subnets based on size: 80, 20, 20, 2, 2, 2. For the 1st one answer ¿how many bits do I require to number 80 hosts?. The answer is 7 bits, because 2^7=128-2=126, if I’d chose 6, I could number only 64-2=62 hosts maximum.  The potential numbering capacity of a network is 2 to the power of the amount of bits in the hosts part, so when I take 7 bits to the hosts part, I’ve got all bits between original network limit and the new limit to number subnets. So I know have 24 begining bits reserved for original network, 1 bit to numbering the new network and 7 bits to number hosts. From here on, the mask is just one, the subnet mask. So for the 80 hosts subnet I need a mask of 25 bits, or /25 or in subnet notation 255.255.255.128. The parameters of this subnet are: subnet address = 192.168.0.0/25; Broadcast address=192.168.0.127/25; assignable to hosts range: from .1 to .126. The next subnet in this scheme should be 192.168.0.128, that marks the next unassigned range without overlap with this subnet, then that’s the address of next subnet but with a longer subnet mask (because should be a subnet of less capacity, that’s why we order the sizes).

The second size in the list is 20 hosts, for 20 hosts I need 5 bits (2^5=32-2=30 hosts), then the subnet address is 192.168.0.128/27 (32-5=27). The broadcast address is the one with all hosts bits in one, 192.168.0.159/27 and the assignable range is .129 to .158. The next subnet with the same mask is 192.168.0.160/27

Because the next subnet have the same size, the subnet mask for the next subnet is the same (/27, the longer mask short enough to count 20 hosts) and from latter subnet we already know which subnet is: 192.168.0.160/27. That is the subnet address and the broadcast address for that subnet is 192.168.0.191/27. Assignable range is from .161 to .190. The next subnet with the same mask is .192/27

The WAN links allow just 2 hosts, so for two hosts I just need 2 bits (2^2=4-2=2) then the mask is 32-2=30. The subnet address is 192.168.0.192/30 for the first link, the assignable addresses are .193 and .194, the broadcast for that link is .195/30. Following the thought: next link address 192.168.0.196/30, hosts .197-.198, broadcast .199/30; and last link 192.168.0.200/30, hosts .201-.202 and broadcast .203/30.

The scheme asked is

Quickly a pattern appears: all the addresses are contiguous, there is still some address space wihtout using at the end of the assignment. Other patterns are those of subnet addresses, range and broadcast addresses. This result should be confirmed in binary, for each subnet, with the next criteria:

• Network address should have the host bits all in zero (the hosts bits are those in zero in the mask)
• Broadcast address should have the host bits all in ones.
• The range should allow, at least, the maximum hosts asked for the subnet
• None of the addresses of a subnet should appear in the range of another subnet.

Another exercise

This is another excercise very similar to this one and you should be able to make more excercies on your own.

From network 192.168.12.0/24, design an addressing scheme based on the following requirements using VLSM:

• Merchandising needs at least 60 hosts
• Sellers are at least 80
• Managers are 20
• And the network have 4 links between routers

The solution is here (Sorry is on spanish). Merchandising = Mercadeo, Sellers = Ventas, Managers = Administrativos, Link = Enlace.

Captions (sorry), for each spanish tag look for an english tag here.

• IP+GW of PC0
• Clock rate for the DCE router in the serial link
• Ping successful in each link, but not from PC to PC: incomplete routing tables
• Interfaces configured = directly connected networks in RT
• Althogh the routing is configured, the routing is not operative yet. The other router shoud be configred also.
• Now both speak RIP
• Sh ip int brief doesn’t show the subnet mask

These are very basic commands, and I wrote this to help my universitary students review the topics they should already know: routing. So, I gave them in the order you should use them, use show running as last options (but before debugging) because in a production network it could give much more information than needed. Finally, remember that routing is a very easy topic in essence, but it gets complex when there are too many routes in the network.

Be aware that this blog has just been installed, so the appearance and style would’t be finnished for a time. Be patient and enjoy the contents.

Before start the main topic I want to define the use I make of the term subnet mask. Subnet mask are 32 bits used to determine which part of an IP address is network part and which is hosts part, in other words, the subnet mask is the way the host knows its subnet address and differentiate the own network/subnetwork from destination network/subnetwork of a packet to send. The mask is then a sequence of ones followed by a sequence of zeros, where the ones are the bits indicating the network part in the IP address. The subnet mask is also referenced by a number, because it’s always a sequence of ones, so you could name a subnet mask by how many ones are at the begining of the mask, e.g. a network mask of 255.255.255.0 is a mask that could be called /24 because there are 24 bits at the beginind set to one followed by 8 bits set to zero (11111111.11111111.11111111.00000000). This slash+number representation of a subnet mask is called subnet prefix and the notation is a / followed by the number of bits in one at the begining of the mask.

Fixed masks

Master VLSM begins by knowing the old method: fixed masks. So, let’s review the topic. A subnet addressing scheme means that I’ll divide the potential numeration capacity of a network (how many IP address can be assigned from that base address pool) into several smaller subnets and the scheme is an enumeration of each smaller subnet address, assignable address range and broadcast address of each particular subnet. Usually the base network mask is a class-based mask, in other words, usually the base mask have 8 bits, 16 bits or 24 bits (255.0.0.0, 255.255.0.0 and 255.255.255.0), those are the subnet mask for class A, Class B and Class C IP addresses respectively.

To ilustrate the idea of creating a scheme with fixed masks, watch the following exercise:

• Base address: 192.168.11.0/24, this is the address given for, say, an ISP. From that address pool we can manage just the last 8 bits(enlarge its length), using the mask we can make subnets.
• New mask: /27 (255.255.255.224): If we lengthen the mask by 3 bits, the new addressing scheme supports 2^3 subnets, using the most significative bits to count the subnets. That leaves 5 bits to enumerate hosts, so we can have 2^5 hosts, but we cannot use the first address (that is the subnet address) nor the last (broadcast address for the subnet), so with 5 bits we have a maximum of 30 hosts per subnet.
• Finally we must enumerate the subnets: First subnet is 192.168.11.32/27, from that the first host address should be 192.168.11.33/27 (mask=255.255.255.224) and the last one is 192.168.11.62/27. The last one 192.168.11.63 is the broadcast address for that subnet and we cannot assign it to any host. If we had to continue numbering the subnets (to complete the scheme), the next one is 192.168.11.64/27, 192.168.11.96/27, 192.168.11.128/27, 192.168.11.160/27, 192.168.11.192/27, 192.168.11.224/27 and for each one we have to give the range of assignable addresses to complete the scheme.

In this technique or scheme, all the subnets have the same subnet mask and because of that the same hosts numbering capacity. That is not flexible and waste addressing space. Additionally, all the subnets are already defined once we choose the mask length.

Using fixed masks, the requirements will be stated in two ways: asking for a number of needed subnets or asking for a minimum capacity of hosts per subnet. In the first case, the assignment is direct: the how many subnets requirement tells me how many bits I need to add to the mask, e.g. for 5 subnets we need 3 bits (2^3=8) so the masks should add 3 bits to allow up to 8 subnets (2 bits gives me only 4 subnets). In the other case, the requirement says the amount of minimum hosts per subnet, I can figure the bits I need for hosts so I have to substract those bits from the hosts part to know how many bits I need for the subnet, e.g. the requirement says the subnets should have at least 20 hosts, so to assign 20 hosts I need 5 bits (2^5=32, including subnet and broadcast addresses so the assignable addresses are 30 hosts maximum) but what I need to know is the subnet mask, if the original network mask is /24 and the hosts part needed is 5 bits then the mask should be /27, because I have 8 bits of hosts in the original network (the bits I can manage) and from those bits at least 5 should remain as hosts part, then rest 3 which I’ll use to subnet. I have to add 3 bits to the original mask /24.

Variable Length Schemes

VLSM solves the stated problems. What I can do with VLSM is using the space just needed, with the fixed mask the space is already assigned once the mask is elected, that means all the subnets are numbered and the range of IP addresses per subnet reserved without possibility of change, even if the hosts aren’t configured yet. In VLSM the reservation of addresses occurs just when we assign a subnet and the remaining space is still available and the capacity of subsequent subnets could be different from the ones already assigned.

Let’s make an example.  Again we take the same base address 192.168.11.0/24, though we have 8 bits to manage and make subnets and assign hosts also.

The  first big difference between Fixed masks and VLSM is that we don’t waste the space, e.g.: if the requirements were subnets of 5, 10 and 30 hosts, using fixed masks the only possible choice is to make the scheme to support subnets of 30 hosts, that means that the other subnets will have an unused capacity of 25 and 20 hosts respectively.

In VLSM we can choose different length masks and so different capacities, e.g. in the example of three networks of 5, 10 and 30 hosts from a class C address (192.168.11.0/24), for the first subnet of 5 hosts we need 3 bits  to numbering hosts (2^3=8-2=6) so we add 5 bits to the subnet mask (/29), then for the 10 hosts subnet we need 4 bits because (2^4=16-2=14 hosts) so the mask should be /28 and for the 30 hosts subnet we need 5 bits (2^5=32-2=30 hosts max.) then the subnet mask should be /27. Once we’ve figured the masks length we can assign the addresses’ ranges. There is a kind of algorithm to subnet with VLSM, the first rule is order the subnets by capacity in decreasing numbers, for our example we put the 30 hosts as the first and the 5 hosts as the last one. Tha’s the order in which we will assign the addresses ranges.

Here  I need to mention several aspects of design that could be arguable. You could assign the subnets beginning with the first subnet possible, which will have the same address that the base network but with a different mask, I prefer to assign the second one leaving a first network available with the same capacity as the first one (the biggest). That behavior is rooted in the old method, fixed masks, back then the rule recommended not to use the first subnet. Another reason I could give is that is a good practice to leave a big subnet available for scalability but all these reasons are (as told before) very arguable. Because this is my example, I’ll make it applyig my preference: leaving the first subnet available.

So, the first subnet usable is the 192.168.0.0/28 which I won’t use but state as a reservoir for future growth. After this subnet I have the 192.168.0.32/28 which I’ll assign to my first subnet, then the range of IP address assignable is from 192.168.0.33 to .62, because the .63 is the broadcast for this particular subnet. Is worthy to mention that the golden rule is that all the assigned ranges should be exclusive, in other words, if the first subnet is assigned the second subnet and subsequent subnets should be after or before the range of the first subnet including their subnet and broadcast address. It’s worthy note also that cisco routers detect this kind of misconfiguration giving an error message.

After the first subnet comes the second, but the second subnet should be smaller, just 10 hosts. Since that, the masks is /29 (24 + 4 more bits because I need 4 to count up to 14 hosts) . Here we must assume the fixed scheme and take the next subnet after the past one with the same mask, 192.168.0.64 but I lengthen the mask by the bits needed, 29 in this case. Then the 2d subnet is the 192.168.0.64 /28, the rest is history: range .65 to .78, with Broadcast .79. If we take any number greater that 79 the hosts would fall into another subnet because the subnet part is different than this range (compare the first 4 bits of the decimal numbers 78 and 81, they are the last assignable address of the subnet .64/28 and the first address of the subnet .80/28).

Finally we need to assign the last subnet, the one with 5 hosts maximum. Again I take the next subnet with the same masks of the previous subnet (192.168.0.64/28) which is 192.168.0.80 but I use the mask I need for 5 hosts, /29, because I need 3 bits to count up to 8 hosts but without subnet and broadcast I can count up to 6 hosts, so from 8 bits I had to manage by my own I substract 3 bits which results in 5 bits and this plus the 24 original bits gives me 29.  The rest is history again: subnet address 192.168.0.80/29, range .81 to .86 with broadcast .87.

Note that with VLSM we still have space, lots of space to assign, even with these 3 subnets  already assigned. We could use the rest of space in subnets of 6 hosts, or 30 hosts until the last subnet range reach the .255 in its broadcast.

Analize it!

And remember that we leave a big subnet at the begining of the space, so we still have a 30 hosts subnet to assign also additionally to the final range.

Let’s analize this result in binary

• 1st subnet’s last byte: 00100000 (.32/27)
• Mask                              11100000
• 2d subnet’s last byte: 01000000 (.64/28)
• Mask                             11110000
• 3d subnet’s last byte: 01010000 (.80/29)
• Mask                             11111000

Special attention to a pattern: the bits in zero in the mask (the hosts part) are the same bits in zero of the subnet address, and even when the last addresses have their first two bits equal, the mask distiguish them.  There is another important consequence of having the hosts part in zero: if we need to assign a big subnet after a little one we could need to leave unnused space. E.g. let’s assume that after we use all this scheme in an organization, we get a new task of assign a subnet of 25 hosts. The requirement says we need a /27 mask because this mask serves well up to 30 hosts and a longer mask don’t, but ¿where to assign the range?. The next subnet after the .80/29 subnet is the .88 but this number doesn’t have the last 5 bits in zero! (make the exercise), then I cannot use this subnet for the new requirement. I need to find a space after the .80 with the last 5 bits in zero, the next space with this condition is the .96/27, converting this number to binary I can see that this do have the last 5 bits in zero, then I can assign this number to the new subnet. By just comparing the ranges I can see also the spare space: subnet .80/29 ranges up to .87 address (from 80 to 87 including subnet and broadcast) and the new subnet from .96 to .127 including subnet and broadcast address, and ¿what about 88 to 95?. Well, I could use this spare space assigning subnets of 6 hosts (3 bits for the host part) or smaller subnets but obviously not for bigger ones.

Finally, there are another interesting consequence of using VLSM. If you watch carefully, the last two subnets have the beginning of their addresses equal up to the 2d bit in the last byte, though, if a single router have this two subnets it could advertise in its routing updates this two subnets as just one, the common part, minimizing the size of the update and easying the routing for uplink routers among other benefits. This last behaviour is called summarization and the common part is called summary route. In routing, the common part is called CIDR which stands for Classless Inter-domain routing.

Conclusion

All this topic is not easy, but the practice makes the difference. I have a final recommendation:

• Make the calculations in binary instead of decimal until you get acquainted with decimal values of binary  numbers.
• Order the requirement from bigger to smaller subnets and assign the biggest first.
• Assign the subnets in sequence as possible.
• If possible, use consecutive subnets in the same router, this is an advanced topic related to summarization.
• Always verify that the range of addresses (including subnet and broadcast) of all the subnets in the scheme be separated in number. None of the possible hosts could be in the same range of two or more subnets.

Everyone knows that links and clicks are the currency of Internet, so I wanted to write this post (which I already wrote in spanish) to atract more people. So I want to get feedback from readers of this post, telling me if it was clear or if I had some mistakes in the writing. Thanks.