Subnet Calculator

One of the biggest challenges to design your local area network and for taking the CCNA exam is calculating subnets.
Actually to calculate subnets is quite easy but it can be confusing for networking newbies. And luckily there are many tools out there that can help us in this problem.

One of the tools is Boson Subnet Calculator, you can download this tool for free from Boson. This thing is very handy, it can calculate subnets, find out which subnet an address belongs to, do a wildcard checking, and have a Decimal IP Calculator.

 

 The only thing you need to do to find what addresses belong to a subnet is just insert the IP address of a computer to the Host IP and slide the Mask bit to match your subnet mask.

As the above picture, I typed in 172.16.10.5 and subnet mask of 255.255.248.0. Immediately I know what addresses for that subnet, you can see them at the Current Host Range.

This tool also valuable to learn subnetting since it shows how it calculates the subnet, recommended for companion to pass CCNA exam.

Too bad you can't use this at the CCNA exam.

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GNS3: Simulator for Your CCNA Exam

When pursuing for the CCNA title, you need to get yourself familiar with the Cisco devices, at least you have tried typing in some commands to configure the Cisco devices.

With a very limited budget, I'm sure it'll be very hard to buy your own Cisco routers and switches, although the prices are quite cheap these days on ebay.

To get around this problem you can use many Cisco simulators that are available out there, one of my favorite is the GNS3.

One problem I found with other simulators is that they don't support all of the commands in the Cisco devices, the GNS3 provides all of them since it is using a real Cisco IOS - the operating system of Cisco devices.

The only drawbacks using GNS3 are:

1. You have to have your own IOS, it's not provided for free from Cisco.
2. It can't simulate switch due to the ASIC hardware used by Cisco switches, but you can use router modules provided by the GNS3 to simulate switches using routers.
   
3. You have to tweak the simulator since it is simulating Cisco devices real time in your PC, it can eat up quite a lot of the PC processor and memory. No worry, you can find how to tweak the simulator in the provided manual from GNS3.

Other than the drawbacks, this simulator is awesome, it's like having your own Cisco devices to play with. Here is the sample of GNS3 interface:

There you can see you can add as many devices as your PC hardware allows, and you can access them through your command prompt telnet function to do the configuration.

Words of note, you can use command prompt telnet function if you use windows XP but in Vista you can't. The reason is Vista doesn't provide telnet function anymore so you have to download third party software for this, many are free by the way.

The coolest function of GNS3 is that if you're connected to the internet, you can simulate your NIC to provide internet connection to GNS3.

You can try configuring internet connection to the routers and test by pinging real live public internet addresses to see if you configure the connection correctly.

You can also install the GNS3 on several PCs and let the simulated routers communicate between the PCs, you can ping between the routers on different GNS3 simulators, and other function too, to keep the processing and memory requirements low, cool.

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TCP and UDP

If you're aiming for the CCNA exam, you definitely have to know about the TCP and UDP protocols. TCP and UDP are protocols that are running on the layer 4 / Host to Host layer / Transport Layer of the OSI Layer.

TCP (Transmission Control Protocol) is what you can say reliable protocol, it numbers each segments that are sent and wait for acknowledgment from the receiving side, when received at the end, the transport layer at that end will assemble the segments and deliver them to the upper layers.

This protocol is said to be connection oriented, it creates a virtual circuit which is a path between the source and destination.
Think of it like making a call, first you pick up the handset and dial a number, this will establish a path and maintain that path until you hung up the phone, same cronology happens at the network world.

Here is what TCP segment format looks like:

In typical CCNA exam question, you'd be asked to differentiate between TCP and UDP segment format, you don't need to remember all the fields up there, after seeing the UDP segment format you'll easily differentiate between TCP and UDP segment format.

UDP (User Datagram Protocol) is a connectionless protocol, it sends all segments to the network without have to wait acknowledgment from the destination host.
There are many reasons to use the UDP, for example in the SNMP used to monitor the network. The SNMP would constantly send messages to update the current status of network devices, using UDP would conserve your bandwidth because the format of the UDP, not to mention if you have to wait acknowledgment for every message you send.

Here is the UDP segment format:

 

Now you know how you can easily differentiate between the TCP and UDP segment format, the UDP has less fields. You can also understand why UDP can conserve the bandwidth in your network.

In my opinion, CCNA exam is a very fair exam, they won't try to trick you by adding false fields in the format, you just need to remember that the UDP segment format has lesser fields than the TCP format.

Related to the CCNA exam also, you need to know which application ports belong to the TCP and which are the UDP's. In some cases, an application would use both protocols such as DHCP.

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How Network Devices Communicate

Getting to know a little deep about how the network devices communicate in your LAN can be quite handy, especially if you're going to take the CCNA exam.

You can find many questions related to the how network devices communicate, what data unit they're using. You have to remember the data unit related to each layer in the OSI Layers.

From the upper layer generate the data, the transport layer turn this data into segments, the network layer divide them into packets, data link layer then divide again into frames, and lastly the physical layer will handle the bits.
You need to be familiar how each device treats the data unit.

Here is the typical question you'd get in the CCNA exam, look at the below picture:

As a network admin you have the above network diagram, you have 2 networks, 192.168.1.0 and 192.168.2.0. Host A sends a request to see email from the Mail Server, what are the destination and source of MAC address and IP address received by the Mail Server?

Now when facing this question we first need to know what will the Host A send to reach the Mail Server.
A Host in a network will only know about its own network, it doesn't know anything about the other network accept for the IP address. So Host A will send the following:

Source MAC : 00-14-BA-AA-AA-AA (the host MAC address)
Destination MAC : 00-14-BA-BB-BB-BB (the router's FA0/0 port MAC address)
Source IP : 192.168.1.2
Destination IP : 192.168.2.2

In term of MAC addresses, remember the host only knows the MAC address of its default gateway (router).
The host will send it to the switch, switch will examine the frame and send it to the port which the router attach to.
Switches always remember the MAC addresses attach to their ports, switches keep this information in its database.

The switch then send it to the router. Router receiving this packet will then examin the packet further. It will check the packet for corruption then strip off the MAC address.
Router will determine where it should forward the packet to, it will first add its own MAC address as the source address and the next hop (router or PC) MAC address as the destination address.
The source and destination IP address will not be altered.

So at last the Mail Server will receive the following information:

Source MAC : 00-14-BA-CC-CC-CC(the router's FA0/1 port MAC address)
Destination MAC : 00-14-BA-DD-DD-DD(the Mail Server's MAC address)
Source IP : 192.168.1.2
Destination IP : 192.168.2.2

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How To Terminate UTP Cable to Toolless Modular Jack

Last post I talked about how to terminate to Modular Jack, it was not hard at all right? There are also modular jacks type that are toolless.
The toolless jacks allow you to terminate UTP cable without the punch down impact tool, though you still have to use cable stripper and scissor, so they're not exactly toolless.

The one that I once tried using before was the one from AMP, lets see if they really cut down the time in terminating the cables.

The tools you need is exactly the same when terminating to regular jacks, but you can exclude the punch down impact tool and add a cutting tools like scissor or cutter.
Here are the steps:

Step One:

Still same with the regular jacks, strip the cable jacket and untwist the cables.

Step Two:

Insert the cables to the holes according to the color codes printed on the jack

 

Now this process takes a little bit more time if this is your first time terminating cables to toolless jacks.

Step Three:

Cut the excess cables using scissor or cutter, it would be easier to use technician scissor.

Make sure you cut them in even length.

Step Four:

Close the cover using your hand, this is quite hard, just close it until you're sure it's firmly closed, and attach the jack to the wall face plate.

 

There you have it, toolless modular jack termination.

In my opinion, I prefer to use the regular jack instead of the toolless ones, I'm more used to terminate to the regular ones and for me, it's harder to arrange the cables in the toolless jacks.

Then again it's up to you to choose, in term of price, the regular and the toolless jacks are not too much different, but the punch down impact tools do cost some good amount of money.

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